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/docs.json

@@ -349,6 +349,14 @@
       "source": "/cron-jobs",
       "destination": "/automation/cron-jobs"
     },
+    {
+      "source": "/cron-vs-heartbeat",
+      "destination": "/automation/cron-vs-heartbeat"
+    },
+    {
+      "source": "/cron-vs-heartbeat/",
+      "destination": "/automation/cron-vs-heartbeat"
+    },
     {
       "source": "/dashboard",
       "destination": "/web/dashboard"
@@ -984,6 +992,7 @@
           "automation/webhook",
           "automation/gmail-pubsub",
           "automation/cron-jobs",
+          "automation/cron-vs-heartbeat",
           "automation/poll"
         ]
       },