feat(calendar): add respond, search, colors, time, conflicts commands

[salmonumbrella]

Summary

Adds comprehensive calendar enhancements inspired by nspady/google-calendar-mcp:

New Commands

• calendar respond: RSVP to calendar events with accepted/declined/tentative status
• calendar search: Full-text search across all calendars with optional date range filtering
• calendar colors: List all available event and calendar colors (useful for --color flag)
• calendar time: Show event times converted to any timezone (e.g., --timezone America/New_York)
• calendar conflicts: Detect scheduling overlaps using the FreeBusy API

Enhanced Existing Commands

• calendar events: Multi-calendar support via --all flag (aggregates all calendars) or comma-separated calendar IDs (e.g., --calendar "primary,work@example.com")
• calendar create: New --organizer flag to set event organizer email, --color flag to set event color ID

Example Usage

# RSVP to an event
gogcli calendar respond --event abc123 --status accepted

# Search for events
gogcli calendar search "team meeting" --from 2025-01-01

# Check for conflicts
gogcli calendar conflicts --from 2025-01-15T09:00:00Z --to 2025-01-15T17:00:00Z

# View events from all calendars
gogcli calendar events --all --max 20

# Show event time in different timezone
gogcli calendar time --event abc123 --timezone Europe/Berlin

# Create event as specific organizer with color
gogcli calendar create "Standup" --start 2025-01-20T10:00:00Z --end 2025-01-20T10:30:00Z \
  --organizer team-calendar@company.com --color 11

Test plan

• All existing tests pass (go test ./internal/cmd/...)
• Added TestParseEventTimes to test the new helper function with edge cases (nil event, nil Start/End, timed events, all-day events)
• Manual testing of new commands with real Google Calendar API

🤖 Generated with Claude Code