* feat: custom heartbeat prompt via YAML config or file
Wire up the existing but unused HeartbeatConfig.prompt field so users
can customize what the agent sees during heartbeats. Adds three ways
to set it: inline YAML (prompt), file-based (promptFile, re-read each
tick for live editing), and env var (HEARTBEAT_PROMPT). Also documents
the <no-reply/> opt-out behavior.
Fixes#232
Written by Cameron ◯ Letta Code
"The only way to do great work is to love what you do." -- Steve Jobs
* test: add coverage for heartbeat prompt resolution
Tests buildCustomHeartbeatPrompt and HeartbeatService prompt resolution:
- default prompt fallback
- inline prompt usage
- promptFile loading
- inline > promptFile precedence
- live reload (file re-read each tick)
- graceful fallback on missing file
- empty file falls back to default
Written by Cameron ◯ Letta Code
"The only way to do great work is to love what you do." -- Steve Jobs
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