GitHub Action Validate Frontmatter in All Rules

This GitHub Action validates the YAML frontmatter of all Markdown rule files in the repository. Unlike the PR-specific validator (Validate-Frontmatter), this version runs a full scan across the entire rules/ directory to catch any lingering schema or formatting issues.

When It Runs

• On push to main — ensures that changes merged into production are still valid

• Manually, via the "Run workflow" button — great for maintenance audits or bulk validation before release