Fix dead links in synced core skills

[Copilot]

Summary

Default synced skills referenced core-only paths that dangle in consumer repositories.

• Replace core ADR and script links with canonical GitHub URLs.
• Correct the frostyard-design cross-skill path.
• Reject skill links that escape .agents/skills/, with regression coverage.

Risk tier

Risk tier: 1 — shared documentation, skills, and validation only.

Docs housekeeping

• New docs started from their category's TEMPLATE.md — N/A
• Every new doc indexed in docs/README.md — N/A
• Cross-links added in both directions (ADR ↔ design ↔ spec ↔ plan) — N/A
• New significant decision recorded as an ADR first, in this PR — N/A
• Conformance aliases (ADR-0029) untouched — canonical targets edited
  instead

Verification

• node scripts/check-docs.mjs green
• Scaffold changed? npm ci && npm test in
  .agents/skills/frostyard-docs-site/scaffold/ green
• Workflows changed? Every action SHA-pinned (40 chars + # vX.Y.Z),
  permissions: {}, persist-credentials: false (ADR-0021) — N/A

• Fixes [core] Synced skills link to core ADRs with repo-relative paths that dangle in all 11 consuming repos #96