Add space caching and auto-resolution for proactive Google Chat messages:
- Auto-cache space IDs when users message the bot (knownSpaces)
- Auto-resolve users/{id} to cached spaces for outbound messages
- Fallback to findDirectMessage API when not cached
- Add comprehensive documentation for proactive messaging patterns
This enables cron jobs, skills, and agents to send proactive messages
to users who have previously interacted with the bot.
Fixes: Users can now send messages via 'users/{id}' target instead of
needing to know the space ID beforehand.
