Change GitHub Actions schedule from 6-hour to 24-hour intervals

Reduced the frequency of newsletter checks from every 6 hours to once every 24 hours (daily at midnight UTC).

Changes:
- Cron schedule: '0 */6 * * *' → '0 0 * * *'
- Now runs once daily at midnight UTC instead of 4 times per day

Rationale:
- Reduces GitHub Actions usage/costs
- Newsletter posts are typically published once every few days, not hourly
- Daily checks are sufficient for timely content updates
- Manual trigger (workflow_dispatch) still available for immediate updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/auto-add-articles.yml

@@ -1,9 +1,9 @@
 name: Auto-Add New Articles
 
 on:
-  # Run every 6 hours
+  # Run once every 24 hours at midnight UTC
   schedule:
-    - cron: '0 */6 * * *'
+    - cron: '0 0 * * *'
 
   # Allow manual trigger
   workflow_dispatch: