docs(arbo): GRANITE_KEY scope now granite + qwen-vision (extended)

The arbo-prompt-enhance vkey was extended to reach qwen3.5-9b-fp8 for the
hero auto-judge step (v0.11.3+), not granite-only. Confirmed via /v1/models
for the key. Docs-only; no version bump.

stacks/arbo/README.md

@@ -34,9 +34,12 @@ the standard fleet pattern:
 - **`ENGINE_TOKEN`** — auth is **OPEN today**. Mint a real bearer
   (`openssl rand -hex 32`), put it in `.env`. Closing this is the one security
   fix the move buys us.
-- **`GRANITE_KEY`** — the LiteLLM **virtual key scoped to arbo** (granite-only).
-  comfy-dev already holds `arbo-prompt-enhance` (issued 2026-06-09); reuse or
-  rotate it. Never the master `sk-corvid`.
+- **`GRANITE_KEY`** — the LiteLLM **virtual key scoped to arbo**. The
+  `arbo-prompt-enhance` vkey (comfy-dev, issued 2026-06-09) was **extended**
+  to reach **both** `granite-4.1-8b` (prompt-craft / `gen` step) **and**
+  `qwen3.5-9b-fp8` (vision / the hero auto-`judge` step, v0.11.3+) — `/v1/models`
+  for this key lists exactly those two. Reuse or rotate it; never the master
+  `sk-corvid`.
 
 ## Q4 — Volumes + backup