feat: ability to trigger work via commands in comments

[ckerr]

Suggested by @clavin here; giving it an issue so that the idea doesn't get lost

Maybe I'm thinking too far head, but question: should this bot be proactive in identifying things it can run in an issue, or should it instead be invoked like a command? The benefit of the latter is that we can easily activate the bot after some back-and-forth, e.g. asking the reporter to create a fiddle.

This could be useful when we need to kick off Fiddle runs manually, the same way we do with Trop when it gets stuck.

[erickzhao]

I think this ended up being our original implementation anyways. Closing.