Add per-calendar sync mode for one-way sync

[MicaelJarniac]

Summary

• Closes Feature request: one-way sync option (read-only source calendars) #34
• Adds a mode field per calendar (read, write, or both) to control sync direction
• read mode: events are sourced from this calendar but no blocker events are written to it
• write mode: blocker events are written here but its events are not read/synced to others
• both mode (default): preserves existing bidirectional behavior — fully backward compatible

Changes

• dbinit.go: Migration v4→v5 adds mode TEXT DEFAULT 'both' column to calendars table
• add.go: Prompts for sync mode when adding a calendar (defaults to both)
• sync.go: Introduces CalendarEntry struct; getCalendarsFromDB returns mode info; syncCalendar skips reading from write-only calendars and skips writing to read-only calendars
• list.go: Shows mode and blocker event count for all calendars (including those with 0 blockers)
• cleanup.go: Updated for new getCalendarsFromDB return type
• README.md: Documents sync modes with a table and usage examples
• CLAUDE.md: Updated architecture docs to reflect new schema and sync flow

Test plan

• Build with go build and verify no compile errors
• Run ./gcalsync add — verify mode prompt appears and defaults to both
• Run ./gcalsync add with read mode — verify events from that calendar create blockers elsewhere but no blockers are written to it
• Run ./gcalsync add with write mode — verify blockers are written to it but its events are not synced
• Run ./gcalsync list — verify mode column is displayed
• Verify existing databases migrate cleanly (mode defaults to both)
• Run ./gcalsync cleanup and ./gcalsync desync — verify they still work

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