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.

tools/process_registry.py

@@ -2812,6 +2812,12 @@ def format_process_notification(evt: dict) -> "str | None":
     if evt_type == "watch_disabled":
         return f"[IMPORTANT: {evt.get('message', '')}]"
 
+    # Overflow events carry their human-readable summary in `message` —
+    # without this case they fall through to the completion formatter and
+    # surface as a phantom "process exited (exit code ?)" notification.
+    if evt_type in ("watch_overflow_tripped", "watch_overflow_released"):
+        return f"[IMPORTANT: {evt.get('message', '')}]"
+
     if evt_type == "watch_match":
         _pat = evt.get("pattern", "?")
         _out = evt.get("output", "")