fix(tools,gateway): format watch_overflow events instead of dropping them

format_process_notification had no case for watch_overflow_tripped /
watch_overflow_released, so a watch-pattern notification flood surfaced
as '[IMPORTANT: Background process  exited (exit code ?)]' — a phantom
exit notification for a process that never existed — while the actual
'watch flood, N notifications suppressed' summary in the event's
message field was silently dropped. The gateway delivery path was
worse: _drain_gateway_watch_events retained only watch_match and
watch_disabled, discarding overflow events entirely before formatting.
Route both event types through the message field in the shared
formatter and the gateway formatter, and retain them in the gateway
drain.

tests/tools/test_watch_patterns.py

@@ -336,3 +336,32 @@ class TestGlobalCircuitBreaker:
                 admitted = True
         assert released
         assert admitted
+
+
+class TestOverflowNotificationFormatting:
+    """watch_overflow_* events must surface their summary, not fall through
+    to the completion formatter as a phantom 'process exited (exit code ?)'."""
+
+    def test_overflow_tripped_formats_message(self):
+        from tools.process_registry import format_process_notification
+
+        evt = {
+            "type": "watch_overflow_tripped",
+            "message": "watch flood detected: 47 notifications suppressed for pattern 'ERROR'",
+            "session_id": "proc_a1b2",
+        }
+        out = format_process_notification(evt)
+        assert "47 notifications suppressed" in out
+        assert "exit code" not in out
+
+    def test_overflow_released_formats_message(self):
+        from tools.process_registry import format_process_notification
+
+        evt = {
+            "type": "watch_overflow_released",
+            "message": "watch flood released: notifications resumed for pattern 'ERROR'",
+            "session_id": "proc_a1b2",
+        }
+        out = format_process_notification(evt)
+        assert "notifications resumed" in out
+        assert "exit code" not in out