Vm: guest agent methods

Method	Returns	Notes
vm.exec(cmd: string, args: List[string])	Result[ExecResult, string]	120 s timeout
vm.exec_timeout(cmd, args, timeout_secs: int)	Result[ExecResult, string]	Custom timeout
vm.copy_to(local: string, guest_path: string)	Result[unit, string]	local relative to lab root; guest path absolute
vm.copy_from(guest_path: string, local: string)	Result[unit, string]	Parent dirs created on host

Exec returns an ExecResult (exit_code, stdout, stderr).