File Watcher

You register watch paths through SessionStart or CwdChanged hooks, then handle changes via FileChanged events. The built-in cwd-changed.js already handles env injection, so this is mainly useful when you want to extend that with additional reactive workflows like auto-reminding to run npm install when package.json changes or restarting TypeScript checks when tsconfig.json updates. Watchers use a 500ms stability threshold to avoid firing on rapid successive changes. Keep your handlers quick since they run synchronously. The matcher syntax is a bit clunky with pipe-separated filenames, but it works.