fix(heartbeat): ignore untouched default template

[yzxlr]

Summary

• treat the generated HEARTBEAT.md template as idle until the user adds tasks below the marker
• stop sending the example/instructions block to the LLM as a real heartbeat prompt
• add regression tests for the untouched-template and user-task cases

Closes #1561

Testing

• go test ./pkg/heartbeat ./pkg/config

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[afjcjsbx]

LGTM

[sipeed-bot]

@yzxlr Smart fix! Treating the generated HEARTBEAT.md template as idle until the user actually adds tasks makes a lot of sense. No more sending boilerplate instructions to the LLM as if they were real prompts.

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