docs(credits): document naming-convention trust in aux free-SKU detector

Mirror the credits_tracker caveat in _is_free_model (a paid stealth/
model would bypass the free_only gate and paid-lane warning) and fix
the stale _warn_paid_lane_once docstring.

agent/auxiliary_client.py

@@ -2830,7 +2830,11 @@ _paid_lane_warned: set = set()
 
 
 def _is_free_model(model: Optional[str]) -> bool:
-    """True when ``model`` is a free SKU (``:free`` suffix or ``stealth/`` prefix)."""
+    """True when ``model`` is a free SKU (``:free`` suffix or ``stealth/`` prefix).
+
+    Naming-convention trust: a paid model shipped under ``stealth/`` would
+    silently bypass both the free_only gate and the paid-lane warning.
+    """
     if not model:
         return False
     normalized = str(model).strip()
@@ -2856,7 +2860,8 @@ def _aux_openrouter_settings() -> Tuple[bool, str]:
 
 
 def _warn_paid_lane_once(model: str) -> None:
-    """Log a WARNING the first time a non-:free OpenRouter model is engaged."""
+    """Log a WARNING the first time a non-free (neither ``:free`` nor
+    ``stealth/``) OpenRouter model is engaged."""
     if model in _paid_lane_warned:
         return
     _paid_lane_warned.add(model)