Step into a room with an invite (open to the innernet team for now)

Rooms are open to the innernet team for now: for anyone else this answers 403 rooms_not_open, before the invite is read. REST only — joining a room is a person's own act, so there is no MCP tool. Requires the write scope. Pass the invite token (or the invite link) a room owner sent. The token is the authorization, as on the web; an invite made for one address only works for that account. a room holds 4 people on the free plan, its owner included. to bring in more, the team plan is opened for it (409 room_full). Joining a room you already stand in is a 200 with already: true.