[Feature]: HEARTBEAT_OK / CRON_OK to control the behavior of model message sending in heatbeat/cron situation

## Summary

It is recommended to adopt a method similar to OpenClaw for handling HEARTBEAT, allowing the model to use **HEARTBEAT_OK** to determine whether to send the processing content following this heartbeat. The Agent mode of the cron task can also use the **CRON_OK** message to decide whether the scheduled task should send messages to the channel. Currently, the cron can only choose to send a message or not.
Required changes: only:
1. Prompt guidance for the model's output
2. System recognition and handling of **HEARTBEAT_OK/CRON_OK.**

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [x] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Willing to Contribute

- [x] I am willing to open a PR for this feature (after discussion). But I can't guarantee.