Add mx_room_create(): bots cannot create rooms

[TroyHernandez]

Problem

The client exposes send/resolve/sync helpers (mx_send_text, mx_send_media, mx_resolve_room, ...) but no way to create a room. Bots can only talk in rooms they're already invited to. Surfaced when tiny was asked to create a cooking room and couldn't; the room had to be created with a raw curl against POST /_matrix/client/v3/createRoom.

Proposal

mx_room_create(client, name, invite = character(), topic = NULL,
               preset = c("private_chat", "trusted_private_chat", "public_chat"),
               encrypted = FALSE, room_alias = NULL)

• Wraps POST /_matrix/client/v3/createRoom
• encrypted = TRUE adds the m.room.encryption initial_state event (megolm)
• Returns the room_id; optionally update the client's room cache
• Expose as a corteza tool so bots can create rooms natively

Workaround

One-off login + curl createRoom (used 2026-07-12 to create the cooking room).