Function step5 

async fn step5<T: Transport, C: Clock>(
    ctx: &mut LauncherCtx<'_>,
    engine: &Engine<T, C>,
    pair_id: &str,
    session_id: &str,
    directory: &Path,
    controller: &str,
    controller_args: &[String],
    server_url: Option<&str>,
    lock: Option<&Path>,
) -> Result<i32, ToolError>

Step 5: the controller, as a child with inherited stdio. Shared with resume, which passes the resume-lock it holds: deleted once claude-pid is written.