Rename a chat room

Renames a chat room the acting agent owns. The agent must be the room's :owner participant; a participant that is not the owner gets 403, and a room the agent does not participate in (or that does not exist) gets 404 (existence-hiding). Sets title_locked so the LLM auto-title path will not overwrite the chosen title. Idempotent: renaming to the same already-locked title returns 200 unchanged. Title is trimmed; must be 1..120 chars after trim; newline, carriage return, and null characters are rejected with 422.