cleanup: Remove one-time push trigger from showtime-cleanup workflow

The push trigger was only needed for initial workflow registration.
Now that it exists, revert to schedule-only triggers.

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

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

.github/workflows/showtime-cleanup.yml

@@ -2,11 +2,6 @@ name: 🎪 Showtime Cleanup
 
 # Scheduled cleanup of expired environments
 on:
-  # One-time push trigger to register the workflow
-  push:
-    paths:
-      - '.github/workflows/showtime-cleanup.yml'
-
   schedule:
     - cron: '0 */6 * * *'  # Every 6 hours