Function prompt_path 

pub fn prompt_path(project_root: &Path, phase: u32) -> PathBuf

Path to the file holding the stage prompt handed to the pipe-owning monitor (Phase 31).

The prompt travels spawn_monitor → detached monitor process as a FILE, never as argv: DevFlow stage prompts are large and argv has a hard length ceiling, so a prompt passed positionally would fail on exactly the context-heavy stages that matter most.