Skill follow-up: markdown-author

## Summary
Address the follow-up actions for the markdown-author skill as identified in the skills review.

## Acceptance Criteria
- [ ] Add example before/after snippets and a simple validation command set.

## Scope
- Applies to `skills/markdown-author/`
- Derived from skills-review.md (commit 9c59dbc)

## Parent
- Refs: #297
