Rather than create forum posts for every single commit, I'd just track the RSS feeds for AF and github.com/nex3/arc, and look for URLs in one that contain URLs in the other.
That's it :) The problem's that it hardly seems worth a new website. Hmm, perhaps I should make it a greasemonkey script running on arclanguage.org that talks to a second server.
It would still be up to committers to describe their change on AF, and to include a link to the commit.
Independent of the webapp I'd like to suggest the best practice of inserting a permalink to a relevant repo at least once during a thread.
Often when I dive into the AF archives I find people talking about variants of arc semantics that it's a lot of trouble to reconstruct. I'm guilty of this as well with wart and whatnot. At the time the context was clear to the participants, but they've since moved on. We could find the context in some nearby thread, but it's hard to locate nearby threads. The only thing that's easy is clicking parent again and again until we get to the top, then doing a search for a github link or something.
