improve changelog/release handling

[jakkdl]

move changelog&version check to pre-commit and to a separate CI job that skips on skip-release label

Remaining quirks:

1. The job won't re-run automatically on label assignment. Claude is telling me I can get there by splitting up the workflows across files, I'll give that a try later.
2. A CHANGELOG run will fail after getting merged to main, because it loses the label. Not sure how to resolve that neatly; maybe just always pass --allow-future if it triggers on push.

The test file is kind of ugly now in that it both looks like a pytest file and doesn't. I should probably add some comments at the very least.

EDIT: actually, I shouldn't skip the CI job with the skip-release label - I should just make that pass --allow-future as well.
(also the flag is somewhat opaquely named, probs time to do a proper argparse solution).

It's also not great that skip-release doesn't actually skip a release attempt, afair we just rely on pushing-the-same-version-again to twine to not do anything.

[jakkdl]

also it doesn't seem to work, at least not when re-running it after adding the label.

[Zac-HD]

I think we've now hit my "not actually worth the trouble" threshold, let's stick with the status quo?

[jakkdl]

The status quo definitely needs changing. I just realized that I forgot to undo the temporary changelog edit in #316 when releasing #317 (and I realized that... as I forgot to update the changelog in #323 before pushing (although that was a different problem of not updating the changelog at all, lel)). But I could look at making a simpler solution

[jakkdl]

okay yeah there's a pretty simple way forward. Let's skip the label, and either have CI always allow FUTURE (but with a check so we don't release); or have CI always error on FUTURE (but it's tested in a separate job that's not required to merge, and main/ will error until we release). Let me know if you have any preferences between those two.

Will keep pre-commit, and fix update_version() that's been silently trying to edit README.md despite the example pre-commit config no longer being there.