Generalize the AI request trigger

[j6k4m8]

A PaperRemote should have its own totally isolated way of identifying a request. (The @ai tag is definitely not the right way to do this; I tried to keep it relatively easy to pluck out, but may have failed.)

This task is bipartite:

• Add a way for a paperremote (maybe in get_next_edit_request) to look for triggers
• Verify that the returned text has no remaining triggers unless they're deliberate (i.e., it's possible that it MIGHT be desired behavior to add a new trigger in the response to chain multiple calls to the AI, but that seems less likely than just accidentally forgetting to strip the comment or whatever)

If someone who is not me wants to tackle this, let's talk about some infra and design — I've started thinking about it a bit.

[j6k4m8]

In Google Docs for instance (#13), it might make sense for the AI to respond to comments or assigned tasks (which I think under the hood is just a comment that starts with @username)

[wrongu]

Closed by #21