GitHub Actions SEO: gate PRs on broken links and schema

Originally published on the Lyra blog. Code review is good at catching logic bugs. SEO bugs are different: a broken canonical does not throw a build error, a dead external link does not fail a type check, and a malformed JSON-LD block does not appear in a diff in any way that signals a problem. They ship quietly. You find out weeks later from Search Console. The fix is a GitHub Actions SEO workflow that gates every blog PR automatically. Four jobs check broken links, meta and canonical correctne