- Add createNotificationService with 10s batch window, urgency mapping, click-to-navigate
- Extend DesktopEvent union with NotificationClickEvent type
- Add 10 unit tests covering batching, urgency, NEVER_BATCH, GC protection, Linux urgency
- Platform-specific: silent flag, Windows timeoutType, Linux urgency level
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>