docs: add heartbeat documentation with spawn/subagent details

- Add Heartbeat section explaining periodic task execution
- Document spawn tool for async subagent creation
- Explain independent context and message tool communication
- Add workflow diagram for subagent communication
- Update workspace layout with HEARTBEAT.md and state/
- Add heartbeat config to config.example.json

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/config.example.json

@@ -100,6 +100,10 @@
       }
     }
   },
+  "heartbeat": {
+    "enabled": true,
+    "interval": 30
+  },
   "gateway": {
     "host": "0.0.0.0",
     "port": 18790