PTY: surface errno from forkpty/write/ioctl failures (T-75)

forkpty failures throw PtyException with the captured errno
(previously a generic StateError). The spawned child's chdir/execve
failures write a diagnostic line to its slave PTY before _exit, so
the parent's reader sees "exec failed: <path>" instead of an
indistinguishable EOF.

NativePty.write and PtySession.write loop on short writes and throw
PtyException on hard errors (with errno). NativePty.resize sets
_dead on EBADF so subsequent calls short-circuit cleanly.

Co-Authored-By: Claude <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-05T13:02:30Z",
+  "exported_at": "2026-05-05T13:07:25Z",
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@@ -9,7 +9,7 @@
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@@ -19,7 +19,7 @@
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       "date": "2026-04-20",
       "file_path": "decisions/architecture.md",
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@@ -29,7 +29,7 @@
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       "date": "2026-04-20",
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@@ -39,7 +39,7 @@
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       "date": "2026-04-20",
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@@ -49,7 +49,7 @@
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       "date": "2026-04-20",
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@@ -59,7 +59,7 @@
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@@ -69,7 +69,7 @@
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@@ -79,7 +79,7 @@
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@@ -89,7 +89,7 @@
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@@ -99,7 +99,7 @@
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@@ -109,7 +109,7 @@
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@@ -119,7 +119,7 @@
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@@ -129,7 +129,7 @@
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@@ -139,7 +139,7 @@
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@@ -149,7 +149,7 @@
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       "file_path": "decisions/extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -159,7 +159,7 @@
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       "file_path": "decisions/extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -169,7 +169,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -179,7 +179,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -189,7 +189,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/accessibility.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -199,7 +199,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/accessibility.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -209,7 +209,7 @@
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       "date": "2026-04-21",
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@@ -219,7 +219,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/testing.md",
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@@ -229,7 +229,7 @@
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       "file_path": "decisions/testing.md",
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@@ -239,7 +239,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/testing.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -249,7 +249,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/testing.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -259,7 +259,7 @@
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       "file_path": "decisions/testing.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -269,7 +269,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/testing.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -279,7 +279,7 @@
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       "file_path": "decisions/testing.md",
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+      "synced_at": "2026-05-05 13:07:03"
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@@ -289,7 +289,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/testing.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -299,7 +299,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/tooling.md",
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@@ -309,7 +309,7 @@
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       "file_path": "decisions/tooling.md",
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@@ -319,7 +319,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/tooling.md",
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@@ -329,7 +329,7 @@
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       "file_path": "decisions/process.md",
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@@ -339,7 +339,7 @@
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       "file_path": "decisions/process.md",
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@@ -349,7 +349,7 @@
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       "file_path": "decisions/process.md",
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@@ -359,7 +359,7 @@
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       "file_path": "decisions/process.md",
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@@ -369,7 +369,7 @@
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       "file_path": "decisions/process.md",
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@@ -379,7 +379,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/process.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -389,7 +389,7 @@
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       "date": "2026-04-21",
       "file_path": "decisions/process.md",
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@@ -399,7 +399,7 @@
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       "file_path": "decisions/architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -409,7 +409,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/tooling.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -419,7 +419,7 @@
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@@ -429,7 +429,7 @@
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@@ -439,7 +439,7 @@
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       "file_path": "decisions/architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -449,7 +449,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -459,7 +459,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
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@@ -469,7 +469,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/architecture.md",
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@@ -479,7 +479,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/architecture.md",
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@@ -489,7 +489,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/architecture.md",
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@@ -499,7 +499,7 @@
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       "file_path": "decisions/architecture.md",
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@@ -509,7 +509,7 @@
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       "file_path": "decisions/architecture.md",
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@@ -519,7 +519,7 @@
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       "date": "2026-04-22",
       "file_path": "decisions/architecture.md",
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@@ -529,7 +529,7 @@
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@@ -539,7 +539,7 @@
       "status": "active",
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       "file_path": "decisions/architecture.md",
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@@ -549,7 +549,7 @@
       "status": "active",
       "date": "2026-04-23",
       "file_path": "decisions/architecture.md",
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@@ -559,7 +559,7 @@
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@@ -569,7 +569,7 @@
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       "file_path": "decisions/tooling.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -579,7 +579,7 @@
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       "file_path": "decisions/tooling.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -589,7 +589,7 @@
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       "file_path": "decisions/tooling.md",
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+      "synced_at": "2026-05-05 13:07:03"
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@@ -599,7 +599,7 @@
       "status": "active",
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       "file_path": "decisions/tooling.md",
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+      "synced_at": "2026-05-05 13:07:03"
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@@ -609,7 +609,7 @@
       "status": "active",
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       "file_path": "decisions/tooling.md",
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@@ -619,7 +619,7 @@
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@@ -629,7 +629,7 @@
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@@ -639,7 +639,7 @@
       "status": "active",
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       "file_path": "decisions/tooling.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -648,7 +648,7 @@
       "title": "Authorisation granularity on the IPC socket",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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@@ -657,7 +657,7 @@
       "title": "Back-pressure on event streams",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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     {
       "id": "Q-3",
@@ -666,7 +666,7 @@
       "title": "Event persistence + audit/undo",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
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     {
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@@ -675,7 +675,7 @@
       "title": "`.canvas` schema compatibility with Obsidian",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-5",
@@ -684,7 +684,7 @@
       "title": "IPC wire-format stability + `schema_version:`",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-6",
@@ -693,7 +693,7 @@
       "title": "Window chrome — native frame vs frameless custom",
       "status": "resolved",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-7",
@@ -702,7 +702,7 @@
       "title": "macOS app bundle signing / notarisation",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-8",
@@ -711,7 +711,7 @@
       "title": "Extension API shape — widgets, subcommands, both?",
       "status": "open",
       "file_path": "decisions/questions-extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-9",
@@ -720,7 +720,7 @@
       "title": "Lua runtime vendoring",
       "status": "open",
       "file_path": "decisions/questions-extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-10",
@@ -729,7 +729,7 @@
       "title": "Extension manifest `schema_version:`",
       "status": "open",
       "file_path": "decisions/questions-extensions.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-11",
@@ -738,7 +738,7 @@
       "title": "Coverage gates — hard thresholds vs soft reporting",
       "status": "open",
       "file_path": "decisions/questions-testing.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-12",
@@ -747,7 +747,7 @@
       "title": "Screen-reader automation (axe-core via Playwright)",
       "status": "open",
       "file_path": "decisions/questions-testing.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-13",
@@ -756,7 +756,7 @@
       "title": "Web production-mode a11y",
       "status": "open",
       "file_path": "decisions/questions-accessibility.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-14",
@@ -765,7 +765,7 @@
       "title": "i18n plurals / gender / date-format tooling",
       "status": "open",
       "file_path": "decisions/questions-accessibility.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-15",
@@ -774,7 +774,7 @@
       "title": "Editor tab — full LSP vs tree-sitter-only highlight",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-16",
@@ -783,7 +783,7 @@
       "title": "`tree-sitter-dart` grammar maintenance",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-17",
@@ -792,7 +792,7 @@
       "title": "Icon set growth",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-18",
@@ -801,7 +801,7 @@
       "title": "Theme hot-reload in release builds",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-19",
@@ -810,7 +810,7 @@
       "title": "(withdrawn)",
       "status": "resolved",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-20",
@@ -819,7 +819,7 @@
       "title": "Kernel DB service — namespaced SQL access?",
       "status": "open",
       "file_path": "decisions/questions-process.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-21",
@@ -828,7 +828,7 @@
       "title": "Pql absorbs planning vs keeps separate",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-22",
@@ -837,7 +837,7 @@
       "title": "Ticket persistence strategy",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-23",
@@ -846,7 +846,7 @@
       "title": "SSH-remote development — run clide against a remote workspace",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-25",
@@ -855,7 +855,7 @@
       "title": "Body text face — mono everywhere vs Josefin Sans UI + mono code",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-26",
@@ -864,7 +864,7 @@
       "title": "Small screen layout (\u003c 1000px)",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-27",
@@ -873,7 +873,7 @@
       "title": "Two-editor split",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-28",
@@ -882,7 +882,7 @@
       "title": "Terminal strip scope — shell only or logs/errors/tests",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-29",
@@ -891,7 +891,7 @@
       "title": "Branch picker location",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-30",
@@ -900,7 +900,7 @@
       "title": "Focus behavior when editor is dirty and viewer is peeked",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "Q-31",
@@ -909,7 +909,7 @@
       "title": "XWayland fallback for frameless — proper Wayland protocol needed",
       "status": "open",
       "file_path": "decisions/questions-architecture.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-2",
@@ -919,7 +919,7 @@
       "status": "active",
       "date": "2026-04-20",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-3",
@@ -929,7 +929,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-4",
@@ -939,7 +939,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-5",
@@ -949,7 +949,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-6",
@@ -959,7 +959,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-7",
@@ -969,7 +969,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-8",
@@ -979,7 +979,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-9",
@@ -989,7 +989,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-10",
@@ -999,7 +999,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-11",
@@ -1009,7 +1009,7 @@
       "status": "active",
       "date": "2026-04-21",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     },
     {
       "id": "R-12",
@@ -1019,7 +1019,7 @@
       "status": "active",
       "date": "2026-04-22",
       "file_path": "decisions/rejected.md",
-      "synced_at": "2026-05-03 20:40:19"
+      "synced_at": "2026-05-05 13:07:03"
     }
   ],
   "decision_refs": [
@@ -2606,10 +2606,10 @@
       "id": "T-75",
       "type": "task",
       "title": "PTY: surface errno from forkpty/execve/write/ioctl failures",
-      "status": "backlog",
+      "status": "in_progress",
       "priority": "high",
       "created_at": "2026-05-05 12:58:59",
-      "updated_at": "2026-05-05 12:58:59"
+      "updated_at": "2026-05-05 13:02:40"
     },
     {
       "id": "T-76",
@@ -2633,10 +2633,10 @@
       "id": "T-78",
       "type": "bug",
       "title": "files.read path traversal — validate paths stay under workspace root",
-      "status": "in_progress",
+      "status": "done",
       "priority": "high",
       "created_at": "2026-05-05 12:58:59",
-      "updated_at": "2026-05-05 12:59:50"
+      "updated_at": "2026-05-05 13:02:34"
     },
     {
       "id": "T-79",
@@ -3933,6 +3933,20 @@
       "old_value": "backlog",
       "new_value": "in_progress",
       "changed_at": "2026-05-05 12:59:50"
+    },
+    {
+      "ticket_id": "T-78",
+      "field": "status",
+      "old_value": "in_progress",
+      "new_value": "done",
+      "changed_at": "2026-05-05 13:02:34"
+    },
+    {
+      "ticket_id": "T-75",
+      "field": "status",
+      "old_value": "backlog",
+      "new_value": "in_progress",
+      "changed_at": "2026-05-05 13:02:40"
     }
   ]
 }

CHANGELOG.md

@@ -27,6 +27,13 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 - Terminal cell grid no longer drifts on bold text — bold rendering
   is suppressed at the painter level since synthetic bold (with no
   Bold.ttf registered) shifts glyph advance widths.
+- PTY surfaces errno on `forkpty`, `write`, and `ioctl` failures
+  instead of swallowing them. `execve` failures in the spawned
+  child now write a diagnostic line to the slave PTY before
+  `_exit`, so the parent's reader sees the cause instead of an
+  immediate EOF that looked indistinguishable from clean exit.
+  PTY `write` loops on short writes; both `NativePty.write` and
+  `PtySession.write` now throw `PtyException` on hard errors.
 
 ### Security
 

lib/src/pty/native_pty.dart

@@ -17,6 +17,9 @@ import 'dart:typed_data';
 
 import 'package:ffi/ffi.dart';
 
+import 'errors.dart';
+import 'ffi/libc.dart' as libc;
+
 // -- structs ----------------------------------------------------------------
 
 final class _Winsize extends ffi.Struct {
@@ -125,6 +128,17 @@ class NativePty {
     final execve = _execve;
     final chdir = _chdir;
     final exit = _exit_;
+    final writeFn = _nativeWrite;
+
+    // Pre-allocate error envelopes the child will write to its stdout
+    // (slave PTY → parent's master fd) before _exit, so the parent's
+    // reader sees a real diagnostic instead of an indistinguishable EOF.
+    final chdirErr = 'clide: chdir failed: $workingDirectory\n'
+        .toNativeUtf8(allocator: malloc);
+    final chdirErrLen = chdirErr.length;
+    final execveErr = 'clide: exec failed: $executable\n'
+        .toNativeUtf8(allocator: malloc);
+    final execveErrLen = execveErr.length;
 
     // Allocate ALL native memory before fork.
     final shellN = executable.toNativeUtf8(allocator: malloc).cast<ffi.Char>();
@@ -153,20 +167,33 @@ class NativePty {
     final pid = _forkpty(fdOut, ffi.nullptr, ffi.nullptr, ws);
 
     if (pid == -1) {
+      // Capture errno BEFORE _freeAll — free() can clobber errno.
+      final err = libc.errno;
       _freeAll(shellN, argvN, allArgs.length, envpN, envList.length, wdN, fdOut, ws);
-      throw StateError('forkpty() failed');
+      malloc.free(chdirErr);
+      malloc.free(execveErr);
+      throw PtyException('forkpty', 'forkpty() failed', errno: err);
     }
 
     if (pid == 0) {
       // CHILD — only pre-resolved FFI calls, no Dart heap.
-      chdir(wdN);
+      // After forkpty(), fd 1 is the slave PTY connected back to the
+      // parent's master fd, so write(1, ...) lands as readable output.
+      if (chdir(wdN) != 0) {
+        writeFn(1, chdirErr.cast(), chdirErrLen);
+        exit(1);
+      }
       execve(shellN, argvN, envpN);
+      // execve only returns on failure.
+      writeFn(1, execveErr.cast(), execveErrLen);
       exit(1);
     }
 
     // PARENT
     final fd = fdOut.value;
     _freeAll(shellN, argvN, allArgs.length, envpN, envList.length, wdN, fdOut, ws);
+    malloc.free(chdirErr);
+    malloc.free(execveErr);
 
     final pty = NativePty._(fd, pid);
     pty._spawnReader();
@@ -240,26 +267,52 @@ class NativePty {
     port.send(null);
   }
 
-  /// Write bytes to the child's stdin.
+  /// Write bytes to the child's stdin. Loops on short writes; throws
+  /// [PtyException] (with errno) on failure. Returns the total bytes
+  /// written, which is always [bytes.length] on success.
   int write(List<int> bytes) {
     if (_dead || bytes.isEmpty) return 0;
     final buf = malloc<ffi.Uint8>(bytes.length);
-    for (var i = 0; i < bytes.length; i++) buf[i] = bytes[i];
-    final n = _nativeWrite(_fd, buf.cast(), bytes.length);
-    malloc.free(buf);
-    return n;
+    try {
+      for (var i = 0; i < bytes.length; i++) buf[i] = bytes[i];
+      var written = 0;
+      while (written < bytes.length) {
+        final n = _nativeWrite(
+          _fd,
+          buf.elementAt(written).cast(),
+          bytes.length - written,
+        );
+        if (n < 0) {
+          final err = libc.errno;
+          if (err == 4 /* EINTR */) continue;
+          if (err == 9 /* EBADF */ || err == 32 /* EPIPE */) _dead = true;
+          throw PtyException('write', 'write to PTY failed', errno: err);
+        }
+        if (n == 0) break;
+        written += n;
+      }
+      return written;
+    } finally {
+      malloc.free(buf);
+    }
   }
 
-  /// Resize the terminal.
+  /// Resize the terminal. Silently no-ops if the fd is already
+  /// closed; flips [_dead] on EBADF so subsequent calls short-circuit.
   void resize({required int cols, required int rows}) {
     if (_dead) return;
     final ws = calloc<_Winsize>()
       ..ref.wsRow = rows
       ..ref.wsCol = cols;
-    _ioctl(_fd, _kTiocsWinsz, ws);
+    final rc = _ioctl(_fd, _kTiocsWinsz, ws);
     calloc.free(ws);
+    if (rc < 0 && libc.errno == 9 /* EBADF */) {
+      _dead = true;
+      return;
+    }
     // Explicitly signal the child to re-query its terminal size.
-    _nativeKill(pid, 28); // SIGWINCH = 28 on macOS/Linux
+    // SIGWINCH = 28 on both macOS and Linux.
+    _nativeKill(pid, 28);
   }
 
   /// Send a signal to the child.

lib/src/pty/session.dart

@@ -161,7 +161,9 @@ class PtySession {
     }
   }
 
-  /// Send bytes to the child's stdin.
+  /// Send bytes to the child's stdin. Loops on short writes; throws
+  /// [PtyException] (with errno) on failure. Returns total bytes
+  /// written, which equals [bytes.length] on success.
   int write(List<int> bytes) {
     if (isClosed) return 0;
     final buf = pkg_ffi.calloc<ffi.Uint8>(bytes.length);
@@ -169,7 +171,22 @@ class PtySession {
       for (var i = 0; i < bytes.length; i++) {
         buf[i] = bytes[i];
       }
-      return libc.write(_masterFd, buf, bytes.length);
+      var written = 0;
+      while (written < bytes.length) {
+        final n = libc.write(
+          _masterFd,
+          buf.elementAt(written),
+          bytes.length - written,
+        );
+        if (n < 0) {
+          final err = libc.errno;
+          if (err == 4 /* EINTR */) continue;
+          throw PtyException('write', 'write to PTY failed', errno: err);
+        }
+        if (n == 0) break;
+        written += n;
+      }
+      return written;
     } finally {
       pkg_ffi.calloc.free(buf);
     }