Add read-only Google Calendar integration via gws CLI

Adds a get_calendar_events agent tool that shells out to `gws calendar +agenda`
for admin users. Controlled by GOOGLE_CALENDAR_ENABLED env var, with OAuth
credentials mounted from credentials.json.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -11,6 +11,7 @@ wheels/
 
 # Environment files
 .env
+credentials.json
 
 # Database files
 database/