feat(log): crash-survivable FileLogSink + dev/prod verbosity toggle (T-432)

First increment of the observability epic (T-425), the productive pivot after
the ConPTY freeze refused to reproduce on CI: if we can't reproduce it, make
the next occurrence leave evidence.

- FileLogSink (lib/kernel/src/file_log_sink.dart): synchronous, crash-survivable
  LogSink. Appends each record as one JSON line to a size-rotated file; fsyncs
  warn/error + risky-source (pty/ffi/conpty/watchdog) records immediately so the
  last breadcrumb is on disk before a hard death, batches the rest on a timer.
  Never throws. Flutter-free → unit-tested under dart test against a temp dir.
- logDirectory() (paths.dart): persistent per-platform log dir (LOCALAPPDATA /
  ~/Library/Logs / $XDG_STATE_HOME) — durable across reboot, unlike the
  ephemeral socketDirectory.
- resolveLogLevel() (log.dart): the requested dev/prod toggle. CLIDE_LOG
  dart-define → CLIDE_LOG env → app.log.level setting → warn(release)/info(debug).
  Lenient parse; an invalid source falls through.
- Boot wiring (facade.boot + main.dart): FileLogSink leads the sink chain (so a
  crash records before the volatile stderr/ring sinks) and the resolved level
  sets Logger.minLevel.

Tests: FileLogSink (JSON shape, error/stack, rotation cap, append-across-restart,
timer-cancel), resolveLogLevel precedence + fall-through, logDirectory per-OS.
Coverage gate 95.11%.

Follow-ups under T-425: live toggle CLI/command/chip (T-433), FFI breadcrumbs
(T-434), watchdog isolate (T-435), CI artifact wiring (T-436).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,15 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ## [Unreleased]
 
+### Added
+
+- **Crash-survivable logging.** clide writes a durable JSON-lines log to a
+  persistent per-platform dir (Windows `%LOCALAPPDATA%`, macOS `~/Library/Logs`,
+  Linux `$XDG_STATE_HOME`), fsyncing warn/error + pty/ffi records immediately so
+  a freeze leaves on-disk evidence. `CLIDE_LOG` (dart-define / env) or the
+  `app.log.level` setting sets verbosity (warn in release, info in debug).
+  (T-432)
+
 ## [2.5.0] — 2026-06-14
 
 ### Added