Shortest integration in PRs

[dgrcode]

I think it would be nice to have shortest interact with a PR review and add/update tests based on the review comments.

A few ways I can see it happening:

• mention shortest in the review message or a particular review comment (maybe @shortest, \shortest, _shortest) to only make changes for the review/comment where the mention is made.
• assign shortest to the PR to go through all the PR comments

Pushing to the open PR is probably not great because it can be hard to see what changes shortest made. Maybe opening a PR against the PR branch is a better approach.
For example, there's a PR main <-- feature_one, then shortest opens a PR feature_one <-- feature_one/shortest. The description should have a reference to the original PR so it shows up there.