ci: run CPython sync weekly instead of daily

Daily syncs keep appending commits to the open sync PR while it is being
reviewed. Run it once a week (Friday 00:00 UTC, 08:00 Asia/Taipei) so
each sync PR is a stable batch to review over the weekend.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: mattwang44

.github/workflows/py315-sync-cpython.yml

@@ -5,7 +5,7 @@ on:
     branches:
       - "3.15"
   schedule:
-    - cron: "0 0 * * *"
+    - cron: "0 0 * * 5"  # weekly, Friday 00:00 UTC (08:00 Asia/Taipei)
 
 jobs:
   sync: