What’s the best current practice if we want to enable agent code review and approval of GitHub PRs but only for specific users?

Maybe code owners? I'm not sure if it'll do exactly what you want; but you can say that "such and such" files must be edited, or reviewed, by specific users.

Then, if someone opens a PR who isn't one of those people listed, it requires them to be a reviewer.

We use it for .editorconfig; specifically, we don't want a newcomer to come in, not like a style rule, and slip in a change.