map POSIX errno to actionable IPC error kinds (T-79)

pane.spawn (via PtyException.errno) and editor.open (via
FileSystemException.osError.errorCode) now route ENOENT to
not_found, EACCES/EPERM to user_error with a permissions hint,
EISDIR/ENOTDIR/EEXIST to distinct user-error/conflict, and
EMFILE/ENFILE to tool_error with a "fd limit hit" hint. The
mapping lives in lib/src/ipc/errno_mapping.dart so other handlers
can adopt the same surface as they pick up errno-bearing failures.

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

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-05T13:12:14Z",
+  "exported_at": "2026-05-06T07:56:04Z",
   "decisions": [
     {
       "id": "D-1",
@@ -2624,10 +2624,10 @@
       "id": "T-77",
       "type": "task",
       "title": "IPC server: per-request timeout, broadcast logging, stale-socket race",
-      "status": "in_progress",
+      "status": "done",
       "priority": "high",
       "created_at": "2026-05-05 12:58:59",
-      "updated_at": "2026-05-05 13:10:57"
+      "updated_at": "2026-05-05 13:12:20"
     },
     {
       "id": "T-78",
@@ -2642,10 +2642,10 @@
       "id": "T-79",
       "type": "task",
       "title": "pane.spawn / editor.open: map errno to actionable error kinds",
-      "status": "backlog",
+      "status": "in_progress",
       "priority": "medium",
       "created_at": "2026-05-05 12:58:59",
-      "updated_at": "2026-05-05 12:58:59"
+      "updated_at": "2026-05-06 07:45:02"
     },
     {
       "id": "T-80",
@@ -3975,6 +3975,20 @@
       "old_value": "backlog",
       "new_value": "in_progress",
       "changed_at": "2026-05-05 13:10:57"
+    },
+    {
+      "ticket_id": "T-77",
+      "field": "status",
+      "old_value": "in_progress",
+      "new_value": "done",
+      "changed_at": "2026-05-05 13:12:20"
+    },
+    {
+      "ticket_id": "T-79",
+      "field": "status",
+      "old_value": "backlog",
+      "new_value": "in_progress",
+      "changed_at": "2026-05-06 07:45:02"
     }
   ]
 }