#20563: fix: skip default HEARTBEAT_OK instructions when custom prompt is set

## Problem When a custom heartbeat prompt is configured via `agents.defaults.heartbeat.prompt`, the system prompt still includes hardcoded HEARTBEAT_OK instructions after the custom prompt. This causes conflicting guidance. **Before (with custom prompt):** ``` ## Heartbeats Heartbeat prompt: <CUSTOM PROMPT> If you receive a heartbeat poll... and there is nothing that needs attention, reply exactly: HEARTBEAT_OK <-- CONFLICTS with custom prompt! ``` ## Solution Only add the default HEARTBEAT_OK instructions when NO custom prompt is configured. Custom prompts should fully define their own heartbeat behavior. **After (with custom prompt):** ``` ## Heartbeats Heartbeat prompt: <CUSTOM PROMPT> ``` **After (without custom prompt - default behavior):** ``` ## Heartbeats Heartbeat prompt: (configured) If you receive a heartbeat poll... and there is nothing that needs attention, reply exactly: HEARTBEAT_OK ``` ## Use Case This enables the "Free Mind Protocol" pattern where agents define custom heartbeat behavior that doesn't require HEARTBEAT_OK responses, allowing for more autonomous agent operation. ## Changes - Modified `src/agents/system-prompt.ts` to conditionally add default HEARTBEAT_OK instructions only when no custom heartbeat prompt is set ## Testing - Backward compatible: default behavior unchanged - Custom prompts now work as expected without conflicting instructions <!-- greptile_comment --> <h3>Greptile Summary</h3> Conditionally adds default HEARTBEAT_OK instructions only when no custom heartbeat prompt is configured. When `agents.defaults.heartbeat.prompt` is set, the system prompt now excludes the default HEARTBEAT_OK instructions, allowing custom prompts to define their own heartbeat behavior without conflicting guidance. - Modified `src/agents/system-prompt.ts:620-633` to wrap HEARTBEAT_OK instructions in `if (!heartbeatPrompt)` check - The `heartbeatPrompt` variable is already trimmed at line 345, so the check correctly identifies when a custom prompt is set - Maintains backward compatibility: default behavior unchanged when no custom prompt is configured - Enables custom heartbeat patterns like "Free Mind Protocol" for autonomous agent operation <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - The change is minimal, well-contained, and logically correct. It adds a simple conditional check that preserves existing behavior when no custom prompt is set, while fixing the conflicting instructions issue when a custom prompt is configured. The variable `heartbeatPrompt` is already properly trimmed at line 345, ensuring the boolean check works correctly for empty strings, undefined, and actual custom prompts. - No files require special attention _{Last reviewed commit: 8447d89} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->