chore: supersede PR #58 (test infrastructure already merged via PR #59)

PR #58 was opened before PR #59 was created. PR #59 bundled the same
3 test-infrastructure commits along with CI/CD overhaul work, and was
merged first. The actual content additions (validate_frontmatter.py,
schemas, smoke test, fixes) are now on dev. This empty commit aligns
the PR branch with dev so the PR can close cleanly as merged.

Author: claude