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>

.claude/settings.json

@@ -74,6 +74,11 @@
             "type": "command",
             "command": "bash .claude/hooks/validate-assets.sh",
             "timeout": 10
+          },
+          {
+            "type": "command",
+            "command": "bash .claude/hooks/validate-skill-change.sh",
+            "timeout": 5
           }
         ]
       }