I just found out that my specific problem with GitHub doesn’t have to do with MY settings at all — I need to ask GitHub to fix an error in their systems.
Hopefully this helps somebody else getting the same problem.
Make sure you can ssh firstname.lastname@example.org
ssh email@example.com ERROR: Hi yuchant! You've successfully authenticated, but GitHub does not provide shell access Connection to github.com closed.
If you are having issues with permissions, read the answer to this stack question: http://stackoverflow.com/questions/922210/unable-to-git-push-master-to-github for tons of debugging ideas.
For me, it turns out it wasn’t my fault : )