stop requiring "sign/" prefix for signing events

[jku]

I originally required "sign/" prefix anticipating slightly more PRs that modify the git repository (but are not actually signing events). That does not seem to happen much as there should be almost nothing else in the repository, and the few that happen we can filter.
So:

• I think we can consider every branch a signing event if there are changes in either targets/ or metadata/ (and branch is not "main" or "publish")
• False positives should be easy for maintainers to handle -- just close the issue or don't create branches like that on the main repository