[Feature] Create `lightspeed-frontmatter-audit` skill

[ashleyshaw]

Summary

Create the first P0 governance skill for auditing AI files for frontmatter, schema drift, stale references, and missing indexes.

Parent epic: #283.

Proposed Solution

Add /skills/lightspeed-frontmatter-audit/SKILL.md with clear workflow steps, inputs, expected outputs, and references to active schemas and validation commands.

Acceptance Criteria

• Skill folder exists with SKILL.md.
• Skill name matches folder name.
• Skill covers frontmatter, schema drift, stale links, and missing indexes.
• Skill output format is documented.
• Skill is eligible for inclusion in plugins/lightspeed-github-ops.
• Documentation/changelog updated.
• PR uses correct branch prefix feat/.

[linear-code]

LS-616

[ashleyshaw]

Completed on 2026-05-26. Added skills/lightspeed-frontmatter-audit/SKILL.md and included plugin packaging path. Validation: npm run validate:skills (pass).