agent_panel: alerts use the macro persona's verdict; all 4 takes still logged

agent_panel.verdict_source defaults to "macro": the macro take's stance is the
verdict, the synthesizer call is skipped (4 LLM calls per panel instead of 5);
"chair" restores the four-voice consensus + OPPOSE guard. Verdict source is
cached per mode and logged (panel_verdict_source). 406 tests.

Claude-Session: https://claude.ai/code/session_01FC4s6VCURFte4dub4xKxYL

signal_aggregator.py

@@ -106,6 +106,7 @@ class SignalReport:
     panel_created_at: str = ""
     panel_cache_status: str = ""
     panel_payload_hash: str = ""
+    panel_verdict_source: str = ""  # "macro" / "chair" / ""
 
     final_signal: SignalStrength = SignalStrength.NEUTRAL
     final_score: float = 0.0