fix: crisis hook only warns on high confidence (#711)

Crisis detection hook logs WARNING only for critical/high severity
(crisis is_crisis=True). Medium/low confidence signals log at INFO
to avoid log noise.

Changes:
- gateway/builtin_hooks/crisis_detection.py: New builtin hook
  that checks user messages on agent:start event
- gateway/hooks.py: Register crisis-detection hook in
  _register_builtin_hooks()

Behavior:
- Critical/high (is_crisis=True) → logger.warning()
- Medium (low confidence) → logger.info()
- No match → no log

Closes #711.

gateway/builtin_hooks/crisis_detection.py

@@ -0,0 +1,51 @@
+"""
+Crisis detection hook — monitors user messages for crisis signals.
+
+Fires on agent:start event to check incoming user messages for crisis
+signals. Only logs WARNING for high-confidence matches (critical/high
+severity). Lower-confidence signals log at INFO level to avoid noise.
+
+Refs: #702, #711
+"""
+
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+async def handle(event_type: str, context: dict) -> None:
+    """Check user message for crisis signals on agent:start."""
+    if event_type != "agent:start":
+        return
+
+    user_message = context.get("user_message", "")
+    if not user_message or not user_message.strip():
+        return
+
+    try:
+        from tools.crisis_detection import check_crisis
+    except ImportError:
+        return
+
+    result = check_crisis(user_message)
+
+    if not result.matched_patterns:
+        return
+
+    # Only log WARNING for high-confidence crisis signals
+    # Lower-confidence matches (medium) log at INFO to avoid log noise
+    if result.is_crisis:
+        logger.warning(
+            "crisis detected: severity=%s lang=%s patterns=%d session=%s",
+            result.severity,
+            result.language,
+            len(result.matched_patterns),
+            context.get("session_id", "unknown"),
+        )
+    else:
+        logger.info(
+            "crisis signal (low confidence): severity=%s lang=%s patterns=%d",
+            result.severity,
+            result.language,
+            len(result.matched_patterns),
+        )

gateway/hooks.py

@@ -66,6 +66,19 @@ class HookRegistry:
         except Exception as e:
             print(f"[hooks] Could not load built-in boot-md hook: {e}", flush=True)
 
+        try:
+            from gateway.builtin_hooks.crisis_detection import handle as crisis_handle
+
+            self._handlers.setdefault("agent:start", []).append(crisis_handle)
+            self._loaded_hooks.append({
+                "name": "crisis-detection",
+                "description": "Monitor user messages for crisis signals (warns only on high confidence)",
+                "events": ["agent:start"],
+                "path": "(builtin)",
+            })
+        except Exception as e:
+            print(f"[hooks] Could not load built-in crisis-detection hook: {e}", flush=True)
+
     def discover_and_load(self) -> None:
         """
         Scan the hooks directory for hook directories and load their handlers.