Add /skill-test suite: linter, behavioral specs, and coverage catalog for 52 skills

- New skill: /skill-test (static | spec | audit modes)
  - static: 7-check structural linter per skill file
  - spec: Claude-evaluated behavioral assertions against test specs
  - audit: coverage report across all 52 skills with priority gaps
- New hook: validate-skill-change.sh — advisory reminder to lint after skill edits
- New template: skill-test-spec.md — standard structure for authoring test specs
- New: tests/skills/catalog.yaml — machine-readable coverage index (52 skills)
- New: tests/skills/_fixtures/ — shared fixtures (complete concept, incomplete GDD)
- New: 4 seed test specs for critical gate skills (gate-check, design-review,
  story-readiness, story-done) — 4 cases each
- Modified: settings.json — validate-skill-change.sh added to PostToolUse hook

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/skills/_fixtures/incomplete-gdd.md

@@ -0,0 +1,51 @@
+# GDD: Light Manipulation System
+
+## Overview
+
+The light manipulation system allows players to interact with bioluminescent
+organisms and ancient light conduits to redirect beams of light. Light beams
+illuminate dark areas, power ancient mechanisms, and reveal hidden surfaces.
+
+## Player Fantasy
+
+The player should feel like a puzzle archaeologist — discovering the logic of
+an alien but internally consistent technology. The "aha" moment when a complex
+light path clicks into place should feel earned and satisfying.
+
+## Detailed Rules
+
+- Players can pick up portable light sources (max 3 carried at once)
+- Stationary conduits redirect beams at fixed angles (45°/90°/135°/180°)
+- Light beams are blocked by solid terrain and most objects
+- Living bioluminescent organisms pulse light on a 3-second cycle
+- Ancient mirrors rotate freely and redirect any light beam that touches them
+- A beam must reach a receptor to activate a mechanism
+
+## Formulas
+
+[SECTION MISSING — not yet authored]
+
+## Edge Cases
+
+[SECTION MISSING — not yet authored]
+
+## Dependencies
+
+- **Oxygen System**: Light sources consume no oxygen but picking them up takes
+  time (opportunity cost with oxygen drain)
+- **Cave Navigation**: Illuminated paths reveal branching routes not visible
+  in darkness
+- Player Inventory System (not yet designed)
+
+## Tuning Knobs
+
+[SECTION MISSING — not yet authored]
+
+## Acceptance Criteria
+
+[SECTION MISSING — not yet authored]
+
+---
+
+*Status: Draft — 4/8 required sections populated*
+*Last updated: 2026-03-13*