Feat: add update and delete support for calendar events

[JonathanGodley]

Summary

• Add update_calendar_event tool — updates title, description, start, end, and/or location on an existing event. Only provided fields are changed; others are preserved.
• Add delete_calendar_event tool — deletes a calendar event by UID.
• Both operations use CalDAV (consistent with existing calendar implementation).
• Includes tests for update (partial update, time update, not-found error) and delete (success, not-found error).
• README updated with new tool documentation.

Test plan

• update_calendar_event preserves unchanged fields when updating title only
• update_calendar_event updates start/end times correctly
• update_calendar_event throws when event not found
• delete_calendar_event deletes event by UID
• delete_calendar_event throws when event not found
• All existing tests pass

[MadLlama25]

Closing as superseded: v1.11.0 ships update_calendar_event and delete_calendar_event via PR #53, which grew out of this work and adds in-place (non-destructive) patching, attendee support, and recurring-event safety on top. Thanks for kicking off the update/delete design here — the tool shapes landed essentially as you proposed them.