Maybe there is an even simpler approach which is to use Mastodon instead of implementing a server. A software project following other software projects is a way to express that they are federated. And people with account on the corresponding forges have control over the Mastodon account of the project. For instance, if I’m the owner of https://github.com/ceph/ceph I can login the Mastodon user matching the project and set it to follow https://mygitlab.com/ceph/ceph. The fedeproxy server is a client of the Mastodon instance and acts by federating https://mygitlab.com/ceph/ceph and https://github.com/ceph/ceph because it reads from Mastodon that the relationship exists.
Which leaves us with much less code to write. Going back to the data model and vocabulary, I’m still inclined to think implementing the “Commit Push” activity is a better first step than “Adding an issue comment”.