By default output is NOT returned — poll read_terminal_buffer to observe
results. Pass wait_s > 0 to sleep that many seconds (capped at 30.0)
after dispatch and get the trailing buffer back in the same call —
ideal for quick commands. Long jobs should still dispatch, then poll.

Hard-wrapped rows are joined and trailing blank viewport padding is
trimmed before the payload is returned.

Args:
    lines: Number of trailing scrollback lines to capture (default 100,
        max 20000).