is:pr state:merged "AI" may be interesting, along with other key words ("claude", "cursor" (too general), etc.). Although other comments in a PR would also get it caught up.
Human language strongly involved, so checking whether AI was used is manual. I'd say this is a secondary kind of check, notified in an unobtrusive way (non-modal <dialog> on the top right?)
This is the most interesting with PRs created by members of an organization or owners of repositories. Although members could be hidden from organizations and I don't know if it looks like a normal unprivileged person or whatever.