Feature Request: shpool attach with no args? Pick the most recent un-attached session

[rlue]

The idea for this convenience feature is taken from tmux: it'd be nice to be able to simply shpool attach (or better yet, shpool a) when there's 1+ unattached sessions and have shpool just pick one to attach to automatically.

[ethanpailes]

This seems like a reasonable feature. We could do this by adding a last_connected_at timestamp to the session table and then issuing a list rpc to choose the most recently accessed session.