Github recently released their Atom editor open-source. Since it's all written in JavaScript, it should be relatively easy to embed. Unfortunately it doesn't look like it runs anywhere except in node and the custom version of Chromium they bundle with Atom releases.
Does anyone know how I would go about packaging Atom up for the browser? Has anyone already attempted this?
Atom.io is an editor that you download, and you can not embed it in a webpage. I think what you are looking for is something like Prose.io. You can sign in with Github and edit files in your repository. Unfortunately, Prose's interface is different than other editors. I am not sure how cleanly it will embed, but it looks like the best place to start. The project is open source, and it was designed to manage GitHub Pages (small static websites hosted on a GitHub repo). I am not sure how well it will integrate with what you are doing, but I hope that gives you a good starting point.