Adds an additive, optional schema extension that future-proofs cross-skill
relationship modeling. All four new fields are optional — every existing
SKILL.md (77/77) passes default validation; --strict warns on missing
optionals (Issue #3 backfills next wave). CI job mirrors the
validate-ci-policy-parity contract; AGENTS.md updated to keep CI parity.
Issue #2 of the DDD+Hexagonal v1 plan. Applies
finding-2026-05-07-ci-parity-as-wave-acceptance.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>