Lint PR titles with GitHub action

The PR title is the commit message that eventually gets added to main, so it would be handy to lint for a conventional-commits prefix with a GitHub action since we don't have any way to enforce it right now if the author edits the title manually. There are some existing actions I've seen that we could maybe use, but did not seem to suit our exact needs at the time -- still worth checking out again before writing our own.

Review existing GitHub Actions that validate pull-request titles and compare their behavior with the repository's conventional-commits requirement before deciding whether one fits. The work is done when an action checks titles, including manually edited titles, and enforces the agreed prefix without requiring a custom action unless existing options are unsuitable.

Written by the indexing model from the issue text.