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/hooks/validate-skill-change.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+# Claude Code PostToolUse hook: Advises running skill-test after skill file changes
+# Fires when any file inside .claude/skills/ is written or edited.
+#
+# Exit behavior:
+#   exit 0 = advisory only (non-blocking)
+#
+# Input schema (PostToolUse for Write|Edit):
+# { "tool_name": "Write", "tool_input": { "file_path": "...", "content": "..." } }
+
+INPUT=$(cat)
+
+# Parse file path -- use jq if available, fall back to grep
+if command -v jq >/dev/null 2>&1; then
+    FILE_PATH=$(echo "$INPUT" | jq -r '.tool_input.file_path // empty')
+else
+    FILE_PATH=$(echo "$INPUT" | grep -oE '"file_path"[[:space:]]*:[[:space:]]*"[^"]*"' | sed 's/"file_path"[[:space:]]*:[[:space:]]*"//;s/"$//')
+fi
+
+# Normalize path separators (Windows backslash to forward slash)
+FILE_PATH=$(echo "$FILE_PATH" | sed 's|\\|/|g')
+
+# Only act on files inside .claude/skills/
+if ! echo "$FILE_PATH" | grep -qE '(^|/)\.claude/skills/'; then
+    exit 0
+fi
+
+# Extract skill name from path (.claude/skills/[skill-name]/SKILL.md)
+SKILL_NAME=$(echo "$FILE_PATH" | grep -oE '\.claude/skills/[^/]+' | sed 's|\.claude/skills/||')
+
+if [ -z "$SKILL_NAME" ]; then
+    exit 0
+fi
+
+echo "=== Skill Modified: $SKILL_NAME ===" >&2
+echo "Run /skill-test static $SKILL_NAME to validate structural compliance." >&2
+echo "====================================" >&2
+
+exit 0