Fix cron schedule: 12-min stagger → 15-min stagger

READMEs state jobs update every 15 minutes. Cron was running
every 12 minutes (5 ticks/hour). Now runs every 15 minutes
(4 ticks/hour) with 2-min offset preserved for stagger.

Author: z-apply

.github/workflows/update-jobs.yml

@@ -2,7 +2,7 @@ name: Update Jobs from Aggregator
 
 on:
   schedule:
-    - cron: '8,20,32,44,56 * * * *'  # Every 12 minutes (staggered schedule)
+    - cron: '8,23,38,53 * * * *'  # Every 15 minutes (staggered schedule)
   workflow_dispatch: # Allow manual triggering
   push:
     branches: [ main ]