Document skills: frontmatter support in reference docs

[Copilot]

The frontmatter reference did not describe how to configure skills: entries, which made this capability hard to discover and easy to misuse. This update adds a focused reference section for skills: and links it to deeper schema and packaging docs.

• Reference coverage

  • Added a new ### Skills (\skills:`)section inreference/frontmatter.md`.
  • Documented supported entry shapes (string and object), SHA pinning requirements, and expression-based refs.

• Auth semantics

  • Documented per-skill auth fields: skill, github-token, and github-app.
  • Clarified that github-token and github-app are mutually exclusive per entry.

• Navigation

  • Added direct links to:
    • /gh-aw/reference/frontmatter-full/ for schema-level detail
    • /gh-aw/reference/dependencies/ for package-based distribution context
  • Updated the “Related Documentation” list to include these references.

skills:
  - mattpocock/skills/tdd@801dca688564c529fa84f247f64472520d9ebe28
  - skill: ${{ vars.IMPECCABLE_SKILLS_REF }}
    github-token: ${{ secrets.GITHUB_TOKEN }}

[pelikhan]

@copilot github actions expression support is removed, do not use those