fix(gsd): only unlink notification lock when owned, prevent foreign lock deletion

_withLock() was unconditionally unlinking the lock file in finally,
even when lock acquisition failed. This could delete another process's
lock and allow unlocked concurrent writes. Now tracks ownership and
only cleans up locks we created.

src/resources/extensions/gsd/notification-store.ts

@@ -275,14 +275,19 @@ function _withLock<T>(basePath: string, fn: () => T): T {
     }
   }
 
+  // Only run the mutation if we actually own the lock
+  const ownsLock = fd !== null;
   try {
-    // Write our PID timestamp into the lock for stale detection
-    if (fd !== null) {
+    if (ownsLock && fd !== null) {
+      // Write our PID timestamp into the lock for stale detection
       writeFileSync(lockPath, String(Date.now()), "utf-8");
       closeSync(fd);
     }
     return fn();
   } finally {
-    try { unlinkSync(lockPath); } catch { /* best-effort cleanup */ }
+    // Only delete the lock if we created it — never remove another process's lock
+    if (ownsLock) {
+      try { unlinkSync(lockPath); } catch { /* best-effort cleanup */ }
+    }
   }
 }