docs(agents): default cron jobs to automation agent

AGENTS.md

@@ -228,6 +228,10 @@ You are free to edit `HEARTBEAT.md` with a short checklist or reminders. Keep it
 
 ### Heartbeat vs Cron: When to Use Each
 
+**Cron agent default:**
+- In direct sessions with Will, use the `automation` agent for cron jobs by default.
+- Only use a different agent if Will explicitly instructs otherwise in that session.
+
 **Use heartbeat when:**
 
 - Multiple checks can batch together (inbox + calendar + notifications in one turn)