Provide an automation hook for opening PRs based on github tags

Something like:
- Given a repository and a revision
- Create a branch called `update/-`
- Run `./add-from-github `
- Make a PR

It would be nice if we could keep most of the logic here. I think we could do that by having it trigger off `repository_dispatch`, and then if someone wants to trigger that from another repository based on whatever condition they like (e.g. "pushed a release tag matching some regex") then they can do that and just send the event over (https://github.com/orgs/community/discussions/26323).