feat: Update daily digest schedule to run at 8:00 JST (23:00 UTC previous day)

Author: futabato

.github/workflows/daily-digest.yml

@@ -2,8 +2,8 @@ name: Daily AI Security Digest
 
 on:
   schedule:
-    # 毎日 9:00 JST (0:00 UTC)
-    - cron: "0 0 * * *"
+    # 毎日 8:00 JST (23:00 UTC 前日)
+    - cron: "0 23 * * *"
   workflow_dispatch:
 
 permissions: