Add event-driven daemon architecture (Phase 1 foundation)
- Add fsnotify dependency for file watching
- Create daemon_debouncer.go: batch rapid events (500ms window)
- Create daemon_watcher.go: monitor JSONL and git refs changes
- Create daemon_event_loop.go: event-driven sync loop
- Add mutation channel to RPC server (create/update/close events)
- Add BEADS_DAEMON_MODE env var (poll/events, default: poll)
Phase 1 implementation: opt-in via BEADS_DAEMON_MODE=events
Target: <500ms latency (vs 5000ms), ~60% CPU reduction
Related: bd-49 (epic), bd-50, bd-51, bd-53, bd-54, bd-55, bd-56
Amp-Thread-ID: https://ampcode.com/threads/T-35a3d0d7-4e19-421d-8392-63755035036e
Co-authored-by: Amp <amp@ampcode.com>
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