GH_REPO and GH_TOKEN are generic enough where you need to worry about other applications making the same mistake, and using the same environment variable name. Especially when asking users to set these permanently, I suggest something like TIL_RB_GITHUB_REPO and TIL_RB_GITHUB_TOKEN. Realistically there's no negative to them being longer because you never need to re-type it after configuration.

Ultimately these should be in their own configuration file, probably under ~/.config, but I'm sure that's being considered.