handle cross-repo workflow runs

We need to work out exactly what to do in the cross-repo commit+push situation.

* There [is a permission for repo owner editing branches of cross-repo PRs](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/committing-changes-to-a-pull-request-branch-created-from-a-fork) and this permission is granted by default!
* However it seems like triggering the action is an extra permission? investigation required
* Can we run the action on OUR repo while still fetching THEIR branch?

It's possible dealing with this will scope-creep some aspect of #4 into the mvp.

(Also going off of what @SaraVieira said, I haven't tested that this is busted myself.)