Receiving a private message doesn't open a new pane for them in the sidebar

I'm not sure if this was intentionally done, but when I received my first PM from somebody it didn't add an entry for them on the left sidebar. It seemed the only way to open one up to see their messages was to *send* them a private message.

If it was intentional, this behaviour means that if you get PM'd and don't notice it in the `!status` window in the session you received it, you won't get any notification about it again.

I think a good fix could be to automatically open a pane on the sidebar for this. Though maybe such a patch should wait until we have a command to actually *close* PMs.

Trace the private-message handling and sidebar update path, using the !status behavior described in the issue as the starting point. Confirm that receiving a PM creates a sidebar pane without requiring a reply, and verify that the message remains discoverable after the receiving session is no longer active.

Written by the indexing model from the issue text.