fix: always log planner subqueries to stderr

The prior pipeline.py only logged the planner outcome when an external
--plan was passed ("[Planner] Using external plan (N subqueries)").
The internal LLM planner and the deterministic fallback ran silently,
so retrieval-breadth failures were invisible without --debug.

After plan finalization, emit a unified trace:

  [Planner] Plan: intent=X, freshness=Y, cluster_mode=Z, subqueries=N, source=external|llm|deterministic
  [Planner]   sq1 label=... search="..." sources=[...]
  [Planner]   sq2 ...

Stderr only; does not touch the user-facing stdout synthesis. The
source= annotation distinguishes --plan (external), provider-backed
(llm), and deterministic paths — so when the 2026-04-19 Hermes Agent
Use Cases failure mode recurs, the trace tells the user which path ran
and what subqueries it produced.

Tests: added test_planner_trace_always_fires_on_mock_run which captures
stderr on a mock pipeline run and asserts the summary + per-subquery
lines appear.

tests/test_pipeline_v3.py

@@ -28,6 +28,30 @@ class PipelineV3Tests(unittest.TestCase):
         self.assertIn("grounding", report.items_by_source)
         self.assertEqual("gemini", report.provider_runtime.reasoning_provider)
 
+    def test_planner_trace_always_fires_on_mock_run(self):
+        """Unit 5: The unified planner trace emits one summary line plus one
+        line per subquery on every run, regardless of --debug. 2026-04-19
+        Hermes Agent Use Cases failure: retrieval-breadth issues were invisible
+        because the internal planner path logged nothing.
+        """
+        import io
+        import contextlib
+        buf = io.StringIO()
+        with contextlib.redirect_stderr(buf):
+            pipeline.run(
+                topic="test topic",
+                config={"LAST30DAYS_REASONING_PROVIDER": "gemini"},
+                depth="quick",
+                requested_sources=["reddit", "x", "grounding"],
+                mock=True,
+            )
+        output = buf.getvalue()
+        self.assertIn("[Planner] Plan: intent=", output)
+        self.assertIn("subqueries=", output)
+        self.assertIn("source=", output)
+        # At least one per-subquery line.
+        self.assertIn("[Planner]   sq1 label=", output)
+
 
 class TestSourceFetchCap(unittest.TestCase):
     """X source fetch count must be capped by MAX_SOURCE_FETCHES."""