feat: add local schedule cache + periodic sync to CalendarScheduler

- New ScheduleCache class: maintains today's full schedule locally
- CalendarBridgeClient.getDaySchedule(): fetch all slots for a date
- Scheduler now runs two intervals:
  - Heartbeat (60s): existing slot execution flow (unchanged)
  - Sync (5min): pulls full day schedule into local cache
- Exposes getScheduleCache() for tools and status reporting

This enables the plugin to detect slots assigned by other agents
between heartbeats and provides a complete local view of the schedule.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

plugin/calendar/index.ts

@@ -31,3 +31,4 @@
 export * from './types';
 export * from './calendar-bridge';
 export * from './scheduler';
+export * from './schedule-cache';