Function session_id_from_capture 

pub fn session_id_from_capture(
    project_root: &Path,
    phase: u32,
) -> Option<String>

Thin file-reading wrapper over claude_session_id: reads the phase’s captured stdout file (via stdout_path) and delegates. None for a missing capture file, never an Err — mirrors evaluate_layer1’s lossy-read convention (CR-01: one invalid UTF-8 byte from raw sh redirection must not silently disable this reader).