Have `login` check for current session about atuin

[ellie]

I'd prefer to go for the approach of having a logout command, and making login bail with a message if there's already a session token.

Checking if the session is actually valid would require us to hit count or some other authed endpoint to ensure validity, but I don't think this is entirely necessary. Users could just logout and then login again if there are any problems (say, we need to roll all session tokens)

[conradludgate]

Personally, I think I'd prefer if it silently exited without logging in again if there's already a session file. Perhaps also providing a logout command to easily delete the session file