Frontmatter Guard Skill Convention: see for citation rules; this skill is structural validation, not citation auditing. Contract This skill guarantees: - Every brain page is scanned against the seven canonical frontmatter validation classes - Mechanical errors (nested quotes, missing closing , null bytes, slug mismatch) are auto-repairable on demand with backups - Validation logic is shared with 's subcheck — single source of truth - Reports per source (gbrain is multi-source since v0.18.0); never silently audits the wrong root Why This Exists Brain pages pile up over months. Agents write the…