Let users drive agents via GitHub

[brillout]

I'm actually increasingly liking this.

Note

Maybe the dashboard isn't that important... (In principle, when I communicate with humans I don't need a dashboard — e.g. I don't have a Suleiman dashboard 😀.) But for showing all tickets and their plans, the dashboard is quite nice.

Ideas:

• Reply to a reviewer's @mention from the run that authored the PR
  
• Show choices via GitHub

[brillout]

Mostly done.