Forward sandbox into agent builder backend

agent.py

@@ -129,11 +129,16 @@ class AgentBuilder(A2AAgent[BuilderConfig, NoAuth]):
         await ctx.emit_progress(
             f"llm: {creds.model} via {creds.source} ({creds.base_url})"
         )
+        try:
+            sandbox = ctx.sandbox
+        except Exception:  # noqa: BLE001 - sandbox is optional in local/MCP runs.
+            sandbox = None
 
         graph = build_agent_builder(BuilderContext(
             bucket=bucket, cp_jwt=cp_jwt,
             project_prefix=f"agents/{name}",
             workspace=workspace,
+            sandbox=sandbox,
             grant_token=getattr(workspace, "_grant_token", None),
             llm_base_url=creds.base_url,
             llm_api_key=creds.api_key,