Use sandbox client for builder validation

agent_builder/skills/agent-quality-review/SKILL.md

@@ -41,8 +41,8 @@ Check these items:
 - `Pricing` contains only `price_per_call_usd`, `caller_pays_llm`, and `notes`.
   Reject `runtime.pricing.compute`, `runtime.pricing.total_usd`, `compute=`, or
   `total_usd=` in `agent.py`; those are derived platform fields.
-- Any code path that builds `ChatOpenAI` checks `ctx.llm.api_key` first or
-  returns a clear setup/config result before the model constructor runs.
+- DeepAgents code uses `create_a2a_deep_agent` or another `ctx.llm`-backed
+  resolver, and checks `ctx.llm.api_key` before model construction.
 - Any use of DeepAgents file tools passes `backend=ctx.workspace_backend()`.
 - Any project skills are seeded into the backend and passed with
   `skills=skill_sources or None`.