gcal: Google Calendar as a CLI and as ix-mcp tools, over one Rust crate

nix run .#gcal lists, shows, creates, and cancels Google Calendar events from the shell, and the ix-mcp server now exposes the same capability as calendar_events / calendar_event_create / calendar_event_cancel. Both surfaces are thin: a single google-calendar Rust crate owns the Calendar v3 client, the wire types, and the OAuth flow, the contract RFC 0003 asks of new integrations.

Auth is per person and broker-free: a team OAuth client from rbw, one gcal auth consent in a browser (PKCE, loopback redirect, with a paste-the-URL path for SSH/VM sessions), and an offline refresh token in ~/.config/gcal/token.json. Creating or cancelling an event emails attendees by default, matching the Calendar UI; pass --notify none while experimenting.