Problem: :CP credentials login/logout/clear is verbose and inconsistent
with other actions that are all top-level (:CP run, :CP submit, etc.).
The clear-all subcommand is also unnecessary since re-logging in
overwrites existing credentials.
Solution: replace :CP credentials {login,logout,clear} with :CP login
[platform] and :CP logout [platform]. Remove the clear-all command and
the credentials subcommand dispatch — login/logout are now regular
actions routed through the standard action dispatcher.