I didn't make it past the technical screener.

The task was to fill in an API for a well-known game. It was enjoyable, and I spent the recommended time on the project, writing tests and a detailed pull request with future steps.

The recruiter called and was very nice but could provide no details as to why my code didn't pass. She invited me to reapply in six months, but without feedback, I have no idea what I would change.

Did I use the "wrong" testing library? Did I do something silly like forget to save a dependency?

No idea.

The idea of doing the screener as a pull request is nice, but the point of a pull request is to provide and respond to feedback. One iteration of feedback and responding to feedback with changes would tell the applicant and GitHub much more than a black-box pass or fail.