ci(mirrors): sync on tags and 3x daily schedule instead of per push

.github/workflows/sync-mirrors.yml

@@ -2,15 +2,17 @@ name: Sync Mirrors
 
 on:
   push:
-    branches:
-      - main
     tags:
       - "**"
   delete:
+  schedule:
+    # 3x daily (08:17/16:17/00:17 UTC+8), off the top of the hour to dodge the
+    # scheduled-run rush. Tags and deletes still sync immediately.
+    - cron: "17 */8 * * *"
   workflow_dispatch:
 
-# Mirror providers reject concurrent forced updates to the same ref. Serialize all
-# branch, tag, and delete events so rapid GitHub pushes cannot race each other.
+# Mirror providers reject concurrent forced updates to the same ref. Serialize tag,
+# delete, and scheduled runs so overlapping triggers cannot race each other.
 concurrency:
   group: sync-mirrors
   cancel-in-progress: false