ironrdp-agent: how to logoff?

`ironrdp-agent disconnect` disconnects from the rdp session, but that session keeps running, so we need a way to logoff. maybe as a `ironrdp-agent disconnect --logoff` or `ironrdp-agent logoff`.

Start at the ironrdp-agent CLI's disconnect command and trace how it ends the RDP connection. Compare the requested --logoff and logoff command behaviors, then verify that the session is logged off rather than merely disconnected.

Written by the indexing model from the issue text.