New releases are tagged and released automatically on merge to main with autotag and the gh CLI tool. See Autotag's README for details of how to annotate commit messages to denote major, minor, or patch version bumps.
The release automation creates the tag and release, but gh cannot actually publish to the marketplace. To update the version available in the GitHub Marketplace, click the edit buttion on the latest release on the "Release" page then on the edit page, check "Publish this Action to the GitHub Marketplace" and then click Update Release.
Any .md file in fixtures/ that begins with good or bad is tested for successful or unsuccessful validation respectively.