Formalize the skill release process with the evaluator as a mandatory gate

[tph-kds]

Context: skills/deckforge-skill-evaluator (added 3.1.0) runs deterministic outcome comparisons between baseline, current, and candidate skills.

Gap: nothing makes evaluator results mandatory before a release, and there is no documented release checklist or version-bump policy.

Acceptance:

• Documented release checklist covering: schema checks, layout audit, output-contract validation, build, behavioral checks, bundle validation, and evaluator results.
• Release gate: a candidate skill cannot ship without a passing evaluator run against core eval cases.
• Version-bump policy (major/minor/patch) with the changelog requirement.
• Process documented in docs/EXTENDING.md or a new release-process doc.