Start a new session's machine ahead of its create

Reserve a Session id (a fresh UUID v4) and start its cloud machine while the prompt is still being written; pass the same id as client_session_id to create within 75 s and the session starts on the warm machine. Advisory and only for enabled workspaces: 204 means nothing was started, and create works the same either way. An unused machine is removed after 90 s.

_{Scope: run}