feat: Add freebusy_query tool (client-side implementation)

[PhilflowIO]

Motivation

FreeBusy queries are useful for "When am I free?" queries.

Problem with native CalDAV free-busy-query

The native CalDAV free-busy-query REPORT (RFC 4791) has poor server support:

Server	Native FreeBusy Support
Google CalDAV	❌ Not supported
iCloud	❌ 500 Internal Server Error
Radicale	❌ Not implemented (Issue #34 open since 2013)
Nextcloud	⚠️ Bugs (Issue #50239, #6740)
Baikal	⚠️ Users report problems
DAViCal	✅ Works

tsdav documentation explicitly warns:

"a lot of caldav providers do not support this method like google, apple. use with caution."

Solution: Client-Side Calculation

Instead of relying on server support, we calculate FreeBusy client-side:

1. Fetch events in time range (works with ALL servers)
2. Evaluate TRANSP property (OPAQUE = busy, TRANSPARENT = free)
3. Calculate and return busy/free slots

Advantages:

• ✅ Works with ALL CalDAV servers
• ✅ Consistent behavior
• ✅ No server dependency

Disadvantages:

• ⚠️ Requires read access to calendar (not just FreeBusy permission)
• ⚠️ More data is transferred

Implementation

• New tool: freebusy_query
• Optional: calendar_url (all calendars if not specified)
• Required: time_range_start, time_range_end
• Optional: include_event_details (shows event titles)