fix(rpc): publish close after registry removal and join watcher shutdown

[code-yeongyu]

What

RPC session close and config-watcher shutdown now observe ownership barriers on every path, including the interleavings that the previous patch-shaped port missed.

• close_session acknowledgements and session_closed events are published only after the session registry has removed the entry. Worker-failure terminals use the same barrier, so an immediate list_sessions never returns the closed session.
• Watch-worker registration checks a shared cancellation flag before and after fs.watch. A shutdown that wins the post-load/pre-registration gap closes the native handle instead of retaining it.
• ConfigReloadWatchEngine.close() cancels synchronously and joins returned disposers; repeated close shares that join.
• Reentrant RPC shutdown() joins in-flight disposal and keeps a failure exit code (serializer-error overlapping stdin EOF).
• Nonpersistent RPC probes do not start OS watchers.

Why

An immediate list_sessions after close must not observe the closed handle, including when the worker fails instead of a clean close. Fire-and-forget watcher unsubscribe during process exit left native streams running into teardown. Snapshot-only RPC probes never needed live watches.

Tests

Seam-driven regressions cover successful close vs grace deadline, worker error/failure frames, queued vs admitted watcher cancellation, reentrant shutdown join, and persistent vs nonpersistent RPC probe watching. Related watch-message assertions now allow the cancellation token field.

Coordination

Does not edit host-lifecycle.ts or host-ensure.ts. Open PR #1662 also edits CHANGELOG.md and packages/coding-agent/src/modes/rpc/changes.md; those changelog hunks may need a combine on merge.

Refs #1656

Plan: .omo/plans/omo-dependency-diet.md

---

Summary by cubic

Fixes RPC session close ordering and watcher shutdown so an immediate list_sessions after close never returns the closed session and native watchers do not outlive teardown.

• close_session acknowledgements and session_closed events now wait for the session registry to remove the entry, including worker-failure terminals; closeMarked still returns at the grace deadline so a stuck worker cannot hang cancel/close.
• Watcher registration checks a shared cancellation flag before and after fs.watch; close() cancels synchronously and joins all disposers, and repeated close shares that join.
• Reentrant RPC shutdown() joins in-flight disposal and preserves a non-zero exit code.
• Nonpersistent RPC probes (no session file) no longer start OS watches.
• Regression tests isolate post-watch cancellation from a later unwatch delivery.
• Changelog files have additions that may conflict with open PR fix(rpc): record connection edges before Windows named-pipe idle exit #1662; combine on merge.

Refs #1656

^{Written for commit dcc0dd2. Summary will update on new commits.}