Phase 8: add the Reference App wiki walkthrough (06_reference_app)

- four pages walking the vet sample: overview, domain & API, pipelines &
  worker, spec-driven client admin
- mkdocs nav section, index + start-an-app cross links
- bundle now exposes the walkthrough as wiki_reference_app_* modules

standards/mcp/index.json

@@ -1,6 +1,6 @@
 {
   "project": "standards",
   "type": "standards",
-  "modules_count": 24,
+  "modules_count": 28,
   "source": "hand-authored"
 }