gamemode-watch: single-instance flock guard

Prevents a manually-launched watcher and the spawn-at-startup one from
racing the same winwatch source; the second instance exits immediately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/gamemode-watch.py

@@ -10,7 +10,9 @@ Launched at niri startup (spawn-at-startup "gamemode-watch"). Reconnects if the
 event stream drops while niri is still alive, and always releases the source on
 exit so gamemode can never get stuck on when the watcher stops.
 """
+import fcntl
 import json
+import os
 import re
 import signal
 import subprocess
@@ -133,7 +135,23 @@ def _on_sigterm(signum, frame):
     raise KeyboardInterrupt
 
 
+def acquire_single_instance():
+    # Hold an exclusive lock so a second watcher (e.g. spawn-at-startup plus a
+    # manual launch) exits instead of racing the same winwatch source. The open
+    # file must stay referenced for the process lifetime to keep the lock.
+    rt = os.environ.get("XDG_RUNTIME_DIR", "/tmp")
+    f = open(os.path.join(rt, "gamemode-watch.lock"), "w")
+    try:
+        fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except OSError:
+        return None
+    return f
+
+
 if __name__ == "__main__":
+    _lock = acquire_single_instance()
+    if _lock is None:
+        sys.exit(0)  # another instance already running
     signal.signal(signal.SIGTERM, _on_sigterm)
     try:
         main()