fix(lotto-signals): pstdev→stdev (ddof=1 sample) + z=None contract 문서화

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent-office/app/curator/signals.py

@@ -7,7 +7,7 @@ signal_runner.py에서 DB 연동 + cron 진입점 담당.
 from __future__ import annotations
 import math
 from dataclasses import dataclass, field
-from statistics import mean, pstdev
+from statistics import mean, stdev
 from typing import Any, Dict, List, Optional, Tuple
 
 
@@ -92,7 +92,7 @@ class AdaptiveBaseline:
 
     @property
     def sigma(self) -> float:
-        return pstdev(self.window) if len(self.window) >= 2 else 0.0
+        return stdev(self.window) if len(self.window) >= 2 else 0.0
 
     def push(self, value: float, draw_no: Optional[int] = None) -> None:
         """FIFO push. window_max 초과 시 가장 오래된 값 제거."""
@@ -108,6 +108,10 @@ class AdaptiveBaseline:
         Returns:
             (z_score, fire_level) — z_score는 cold start/warmup이면 None.
             fire_level ∈ {'warmup', 'noop', 'normal', 'urgent'}
+
+        NOTE: z_score is None when sigma==0 (degenerate window) or warmup.
+        Callers must treat None as "signal present but unquantified" — do not
+        compare None with thresholds directly.
         """
         if self.size < 4:
             return None, "warmup"