Cool! That is really interesting. ActivityPub would be great for this. I've added in links to the issues below:

Gitea:

- https://github.com/go-gitea/gitea/issues/1612

- https://github.com/go-gitea/gitea/issues/184

Gogs:

- https://github.com/gogs/gogs/issues/4437

Gitlab:

- https://gitlab.com/gitlab-org/gitlab-ce/issues/44486

- https://gitlab.com/gitlab-org/gitlab-ee/issues/4517

edit: added gitlab enterprise link (plus formatting)

ActivityPub is indeed both simple and extensible. Perfect for issues and comments, also PRs.

For the actual code behind PR I would just setup a git daemon and reference own repo. With appropriate CORS the code can even be cloned in a browser.

And (maybe as a result of this discussion.. who knows) this has just been created:

https://github.com/git-federation/gitpub

Crazy, I was learning about ActivityPub yesterday. The vocab could theoretically support this over torrents, too (although I haven't yet thought through how it would work as a protocol):

    {
        "@context": {
            "@vocab": "https://www.w3.org/ns/activitystreams",
            "git": "https://gittorrent.org/ns/"
        },
        "type": "git:PullRequest",
        "to": ["gittorrent://cjb/gittorrent#master"],
        "git:from": "gittorrent://zamalek/gittorrent#feature/activitystreams"
    }