Add sol call calendar CLI for user-created calendar events
- Add CalendarEvent data model and EventDay container with JSONL CRUD and file locking (apps/calendar/event.py)
- Add calendar CLI commands: create, list, update, cancel (apps/calendar/call.py)
- Extend get_events() to merge user calendar events with AI events
- Extend get_month_event_counts() to include user calendar event counts
- Add formatter registry entry for calendar JSONL files
- Add fixture calendar JSONL files and comprehensive unit tests