Gap closure: feedback loops, traceability, and new /content-audit skill

- NEW /content-audit skill: GDD-specified content vs implemented content gap
  report with COMPLETE/IN PROGRESS/EARLY/NOT STARTED per system
- balance-check: Fix & Verify Cycle phase (fix → re-verify → propagate-design-change)
- perf-profile: Scope & Timeline Decision phase for M/L effort optimizations
- playtest-report: Action Routing phase categorizes findings → design/balance/bugs/polish
- review-all-gdds: Phase 4 Cross-System Scenario Walkthrough (multi-system sequences)
- story-done: Test-Criterion Traceability (each AC mapped to a test, BLOCKING if >50% untested)
- code-review: ADR Compliance Check (ARCHITECTURAL VIOLATION / ADR DRIFT / MINOR DEVIATION)
- setup-engine: upgrade subcommand (pre-upgrade API scan, migration plan, VERSION.md update)
- story-readiness: Asset References Check (verifies referenced asset paths exist)
- validate-assets.sh: invalid JSON now exits 1 (blocking); naming issues exit 0 (warning)
- workflow-catalog.yaml + sprint-plan: /scope-check wired into production phase

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

.claude/skills/story-readiness/SKILL.md

@@ -144,6 +144,22 @@ items pass or are explicitly marked N/A with a stated reason.
   story that depends on a DRAFT or missing story is BLOCKED, not just
   NEEDS WORK.
 
+### Asset References Check
+
+- [ ] **Referenced assets exist**: Scan the story text for asset path patterns
+  (paths containing `assets/`, or file extensions `.png`, `.jpg`, `.svg`,
+  `.wav`, `.ogg`, `.mp3`, `.glb`, `.gltf`, `.tres`, `.tscn`, `.res`).
+  - For each asset path found: use Glob to check whether the file exists.
+  - If any referenced asset does not exist: **NEEDS WORK** — note the missing
+    path(s). (The story references assets that have not been created yet.
+    Either remove the reference, create a placeholder, or mark it as an
+    explicit dependency on an asset creation story.)
+  - If all referenced assets exist: note "Referenced assets verified:
+    [count] found."
+  - If no asset paths are referenced in the story: note "No asset references
+    found in story — skipping asset check." This item auto-passes.
+  - This is an existence-only check. Do not validate file format or content.
+
 ### Definition of Done
 
 - [ ] **At least 3 testable acceptance criteria**: Fewer than 3 suggests