UNDO now prompts "Enter the number of operations to undo or
[Auto/Control/BEgin/End/Mark/Back] <1>" and undoes that many; Enter undoes
one. Its other options say they are not available yet (Auto and Control
still take their value), so nothing that follows is run as a command.
The JSON form (`undo {count?}`) is unchanged.
U is its own command, as in AutoCAD: it undoes one operation without
asking and is what Edit > Undo, Cmd+Z and the toolbar button run.
An interactive command outside undo that changed the drawing (UNDO)
records no undo step when it finishes, matching `execute`.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
