At some point my blog had a working theme. I tried to add a remote theme and it never worked, so I had removed the theme from my _config.yml entirely. I just revisited to try to out themes again, but now I can’t get anything to build at all. I tried reducing the file to nothing, or gone entirely, or just with a theme added via the configurer, and all produce a generic error & email:
The page build failed for the `master` branch with the following error: Page build failed. For more information, see https://help.github.com/en/github/working-with-github-pages/troubleshooting-jekyll-build-errors-for-github-pages-sites#troubleshooting-build-errors.
I assume I corrupted something, but I have no idea what.