I’m not sure why this couldn’t function as oauth provider for HN. Why does it support a new flow?

Yup that's the hope/intention -- If you have an app I can add you as an OAuth2 App (the registration page isn't up yet, but it will be soon)!

Is the author a literal yak shaver, á la sheep-shearer, but for yaks? (Or is it perhaps another way of saying "I can't legally tell you what I do for a living"?)

It's a reference to https://en.wiktionary.org/wiki/yak_shaving

The author is saying "Yes, I know I've built something that's not very useful here"

Thank you! I'd never heard that particular phrase before.

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(Ignore the mailing list bit). If you're able to log in there's a more direct form at the end of the flow!