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/minimal-game-concept.md

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+# Game Concept: Echoes of the Deep
+
+## Overview
+
+Echoes of the Deep is a single-player atmospheric puzzle-platformer set in
+a bioluminescent underwater cave network. Players control a deep-sea diver
+exploring ancient ruins while managing oxygen supplies and manipulating light
+sources to reveal hidden paths and solve environmental puzzles.
+
+## Player Fantasy
+
+The player should feel like a lone explorer uncovering a lost civilization,
+experiencing wonder at beautiful environments, and the satisfying "aha" moment
+when a clever puzzle clicks into place. The oxygen mechanic creates gentle
+pressure without punishing failure harshly.
+
+## Core Loop
+
+1. **Explore** — navigate branching cave sections using light and movement
+2. **Discover** — find oxygen caches, light sources, and ancient mechanisms
+3. **Solve** — manipulate light and environment to unlock new areas
+4. **Progress** — unlock deeper cave sections with escalating complexity
+
+## Game Pillars
+
+1. **Wonder** — every area should contain something visually or mechanically surprising
+2. **Accessibility** — the game should be completable without frustration; oxygen
+   manages pacing, not punishment
+3. **Environmental Storytelling** — the ruins tell a story without text exposition
+
+## Target Audience
+
+Casual-to-midcore players who enjoy relaxed exploration games (Subnautica,
+Journey, ABZÛ) and puzzle games that reward observation over reflexes.
+Target age: 16+. Target sessions: 30–90 minutes.
+
+## Unique Selling Points
+
+- Bioluminescent light manipulation as the core puzzle mechanic
+- No enemies — tension comes from environment and resource management
+- Procedurally decorated (handcrafted levels, procedural detail pass)
+
+## Technical Scope
+
+- **Engine**: Godot 4.6
+- **Platform**: PC (Steam), with console ports post-launch
+- **Team size**: Solo developer
+- **Target completion**: 12-month development cycle
+- **Scope**: 4–6 hours main story, 8–12 hours completionist
+
+## Art Direction
+
+Darkly atmospheric with vibrant bioluminescence providing the primary color
+palette. Deep blues, purples, and blacks punctuated by greens, teals, and
+ambers from living organisms and ancient technology.
+
+## Fun Hypothesis
+
+Players will feel rewarded by the combination of visual beauty and the
+satisfying moment of discovering how light manipulation solves each puzzle.
+The oxygen system will create just enough pressure to make exploration feel
+meaningful without making death feel punishing.