Hm, is that checkable?

At the very least, the first or at least one commit message in a PR can be checked to begin the regex ^[.+].

Which system is able to query and process the git metadata, though? GitHub Actions?

Also, I'm not sure we really should have a zero-tolerance check here; there are cases where that format isn't appropriate. It's just that most commits should follow that format.

Which system is able to query and process the git metadata, though? GitHub Actions?

I believe so.

Also, I'm not sure we really should have a zero-tolerance check here; there are cases where that format isn't appropriate. It's just that most commits should follow that format.

Perhaps a more appropriate solution would be to use templates as a reminder for this most-common case, and a link to the README for completeness. For this, the "Get involved:" should be a header so that it can be linked to.

@tkoeppe, there's always the chance to manually override a failed check and merge regardless. Personally, I think an automated check seems worthwhile.