> I don't know if I've ever committed a file unintentionally since adopting it.

I’ve had the opposite problem: forgetting to add new files.

> I like it especially in concert with git commit --amend, which lets me tack my newest changes onto the previous commit. (Though an interactive rebase with fixup is even better)

No need for the rebase to be interactive:

    $ git commit --fixup=<commit>
    $ git rebase --autosquash <base>

> I’ve had the opposite problem: forgetting to add new files.

Any good solutions for this around?

For now I've adopted running `git status` after `git add -p` to make sure there's no untracked files, but it feels a bit clunky

I occasionally forget to add a new file but don't mind it much. I consider it a significantly smaller problem than committing a file that shouldn't be. CI is gonna run and my tests are surely gonna fail if I didn't commit some file. So I'll see that and commit --amend or fixup to add the new file.

unless the file I forgot to commit is the tests, which hopefully I'll catch by the time of the PR

You can run the tests on the actual produced commit, if you missed some files there would be a compilation error.