Function ssh_pipe_file 

pub fn ssh_pipe_file(
    spec: &SshSpec,
    remote_path: &str,
    bytes: &[u8],
    mode: u32,
) -> Result<()>

Stream bytes to remote_path on the host, then chmod mode. Used for the binary, config.toml, and secrets.toml — the proven pattern of piping over the same control-channel ssh rather than requiring scp.