plan(SP22): Task 4 review fix — client() fixture must enter async context

The verbatim plan returned an un-entered CycloneClient from the fixture,
causing all 3 tests to fail with 'use async with CycloneClient(...)'.
Convert fixture to async generator that enters the context. Verified by
running pytest tests/test_api_client.py -v → 3 passed.

docs/superpowers/plans/2026-06-21-cyclone-pipeline-agent.md

@@ -645,8 +645,9 @@ from cyclone_pipeline.api_client import CycloneClient, HealthSnapshot, Scheduler
 
 
 @pytest.fixture
-def client():
+async def client():
-    return CycloneClient(api_base="http://test:8000")
+    async with CycloneClient(api_base="http://test:8000") as c:
+        yield c
 
 
 @pytest.mark.asyncio