FR: shpool should have a flag to disable threading exit status back

Sometimes the exit status of the shell is what is interesting, and sometimes a better question to answer with the exit status is "was the attach successful". This latter question can use useful if you want to automatically run shpool attach when you shell into a remote machine and display an error if it is not successful. Threading the error status back down causes problems for these sorts of setups.

I'm not sure what to call the flag. Maybe --error-status-mode attach, where the options are attach and shell (with shell as the default to maintain current behavior).

Start by tracing the shpool attach command and how the shell's exit status is currently propagated. Compare the proposed attach and shell modes, then identify the CLI parsing, status-handling, documentation, and test locations involved. Done means a flag selects attach-success status without changing the default shell-status behavior.