`session attach` is confusing, should probably be called `status`

[plotnick]

We don't really "attach" to sessions in the same way we do to jobs, so the command name is somewhat confusing. Consider renaming or at least giving attach an alias status, and advertise that as the default session sub-command.

[plotnick]

A better default might be: attach or ask to attach if a session exists, start a new one if not.