docs(conventions): dev-first framing + flag-enable + accuracy sweep

The conventions standard is dev-first by design — the developer receives the
architecture map + per-task constraints at spawn and owns conforming code from
the start; QA and the PR reviewer are the downstream net. Make that explicit in
the developer prompt and add a dedicated conventions section to the RAG
developer doc (it previously only mentioned the gate, reactively).

Also reconcile the docs with the hardened behavior: env-reference now shows the
flag is off by config default but on in the compose orchestrator block (left off
in the registry), mirroring toolchain matching; and the RAG standard's example
comment no longer implies a misplaced helper blocks (it warns).

docs/rag/standards/conventions.md

@@ -21,7 +21,7 @@ languages: [python, typescript]
 modules:
   - path: app/routers
     purpose: HTTP routing only
-    forbidden: [model, helper]      # no Pydantic models or helpers in routers
+    forbidden: [model, helper]      # a model here blocks; a helper only warns
   - path: app/models
     purpose: data models
   - path: app/services