docs: add cron vs heartbeat decision guide

- New docs/automation/cron-vs-heartbeat.md with complete guidance
- Cross-links from heartbeat.md and cron-jobs.md
- Updated AGENTS.md template with practical guidance
- Added navigation entry in docs.json

docs/reference/templates/AGENTS.md

@@ -112,6 +112,23 @@ Default heartbeat prompt:
 
 You are free to edit `HEARTBEAT.md` with a short checklist or reminders. Keep it small to limit token burn.
 
+### Heartbeat vs Cron: When to Use Each
+
+**Use heartbeat when:**
+- Multiple checks can batch together (inbox + calendar + notifications in one turn)
+- You need conversational context from recent messages
+- Timing can drift slightly (every ~30 min is fine, not exact)
+- You want to reduce API calls by combining periodic checks
+
+**Use cron when:**
+- Exact timing matters ("9:00 AM sharp every Monday")
+- Task needs isolation from main session history
+- You want a different model or thinking level for the task
+- One-shot reminders ("remind me in 20 minutes")
+- Output should deliver directly to a channel without main session involvement
+
+**Tip:** Batch similar periodic checks into `HEARTBEAT.md` instead of creating multiple cron jobs. Use cron for precise schedules and standalone tasks.
+
 **Things to check (rotate through these, 2-4 times per day):**
 - **Emails** - Any urgent unread messages?
 - **Calendar** - Upcoming events in next 24-48h?