Make (or return) a room for a group

Creates an ordinary encrypted group chat holding the owner and the group's agents and records it as room_chat_id; a second call returns the same room (created: false). Grouping never forces messaging -- this is the explicit step. Callable by a delegate under an agent.groups mandate (X-Salt-Act-For), who acts on the GRANTOR's groups. A delegate-made room holds tree agents only.