I am wondering if there is a simple way to use github's api to author commits from the frontend with just a github username and password (or personal auth token with the right repo permissions)?

yes but due to how oauth works, you should not expose the backchannel secrets used to verify a user. Thus you need to provide two endpoints and a configuration change in order for your custom setup to work.

With NextJS and vercel/cloudflare-pages/etc this is as trivial as adding the the below setup and configuring some server side secrets

👉🏻 https://gist.github.com/airtonix/293705c3d67ed6735f0e5e540d73b67e

edit: also noticed the "GitHub Backend" page

It'd be great if the the above gist was officially a monorepo package from @decap/github-oauth-routes (or something)