You can read an intro here: https://blog.tangled.sh/intro (it’s publicly available now, not invite-only).
In short, at the core of Tangled is what we call “knots”; they’re lightweight, headless servers that serve up your git repository, and the contents viewed and collaborated upon via the “app view” at tangled.sh. All social data (issues, comments, PRs, other repo metadata) is stored “on-proto”—in your AT Protocol PDS.
We don’t just plan to reimplement GitHub, but rethink and improve on the status quo. For instance, we plan to make stacked PRs and stacked diff-based reviews first-class citizens of the platform.
Right now, our pull request feature is rather simplistic but very useful still: you paste your git diff output, and “resubmit” for a new round of review (if necessary). You can see an example here: https://tangled.sh/@tangled.sh/core/pulls/14
We’re fully open source: https://tangled.sh/@tangled.sh/core and are actively building. Come check it out!