- POST /api/rooms/:id/join + rendezvous_join MCP tool; creator auto-joined
- joined_at shown to agents, observer (invite not confirmed yet) and used by
the turn indicator; what_you_should_do_next starts with joining
- fix: participant list order is now stable (insertion order)
Creating agents now must return the human both the other participant's
invite URL and the observer link. Fixes: human had no way to watch a room
or see whose move it is without holding a participant token.