fix(watcher): exclude ignored dirs before watching to prevent inotify exhaustion (#276)

The file watcher registered a recursive watch over the entire project (node_modules, build output, caches included) and filtered only in the callback — exhausting the Linux inotify budget on large repos (#276). It now uses chokidar and excludes the same directories the indexer ignores (built-in default-ignore set + the project .gitignore) BEFORE registering a watch, so the watch count on a 900-dir node_modules drops from ~1200 to ~14 even with no .gitignore. Stacks with the shared daemon (#411): one watcher across agents, now small.

Also hardens the #411 daemon lockfile against a concurrent-startup race the new watcher timing made reproducible — the lock is now created atomically with its content (temp-write + hard-link), so racing daemons can never both win. Validated on macOS, Linux (Docker), and Windows (chokidar + fs.linkSync on NTFS).

Co-Authored-By: Colby McHenry <me@colbymchenry.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>