GitHub authentication failing over https, returning wrong email address

GitHub's support determined the root of the issue right away: Two-factor authorization.

To use GitHub over the shell with https, create an OAuth token. As the page notes, I did have to remove my username and password credentials from Keychain but with osx-keychain in place, the token is stored as the password and things work exactly as they would over https without two-factor authorization in place.

I do not have an @github.com address

You don't have to: the @ is the separator between the username:password and the domain.
It is not an email address.

A full GitHub https url would be:

https://username:password@github.com/username/reponame.git

Without the password (which would then be asked on the command line), that would gave:

https://username@github.com/username/reponame.git

But again, username@github.com isn't an email address, just the first part of the credentials.

Make sure the case of your username and reponame is correct: it is case sensitive.

Note that you can store and encrypt your credentials in a .netrc.gpg (or _netrc.gpg on Windows) if you don't want to put said credentials in clear in the url.
See "Is there a way to skip password typing when using https://github".

• Go to Credential Manager => Windows Manager
• Delete everything related to tfs

• Now click on Add a generic credential and provide the following values

  (1) Internet or network adress: git:https://tfs.donamain name(2) username: your username(3) password: your password

  this should fix it