I think I got it. git rebase --onto main just rewound the HEAD revision, but then GitHub Desktop told me I had commits to push and pull; once I did that, everything looks peachy. (Maybe leaving out --onto would have made it one step...)
git rebase --onto main
Now, once GitHub changes GitHub Pages to work from the default branch (which I imagine has to be something they'll do soon, since they're all in on this), I can completely remove it.
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