fix(agent-core): release filesystem watchers on session close

[morluto]

Related Issue

No linked issue. This is a focused follow-up to #1276 for one session-owned resource type.

Problem

Filesystem watchers are session-scoped resources. Each session watcher owns path references, connection/session reference maps, pending change buffers, debounce timers, and an underlying chokidar watcher.

Today, FsWatcherService can create watcher state for a session, but it does not release that state when ISessionService.onDidClose fires. As a result, when a session closes while the service remains alive, watcher state for that session can remain until paths are explicitly removed or the whole service is disposed.

This breaks the expected ownership model: closing a session should release resources owned by that session.

Code-path proof:

1. FsWatcherService.addPaths() creates or reuses a SessionEntry in sessions.
2. Each SessionEntry owns path references, connection references, pending change state, timers, and a chokidar watcher.
3. FsWatcherService.removePaths() and service disposal clean entries, but session close did not clean entries.
4. ISessionService exposes onDidClose, but FsWatcherService did not subscribe to it.
5. Therefore, closing a session did not release filesystem watcher state owned by that session.

What changed

FsWatcherService now subscribes to ISessionService.onDidClose and disposes watcher state for the closed session.

The cleanup removes the session from each connection map, drops empty connection maps, and disposes the session watcher entry.

A focused regression test verifies that closing a session releases the watcher, clears watched paths for each connection, and resets connection path counts.

Validation

• pnpm --filter @moonshot-ai/server exec vitest run test/services.test.ts passes: 1 file, 22 tests.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 187e78e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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