[ai-upgrade] ESC to move back during prompt (#99319)
Stacked on #99291.
### Why?
Let users revisit upgrade choices before the new agent starts.
### How?
Use the existing CLI selector with a dim hint above each menu. Esc goes
back to the previous choice, or cancels from the first prompt; Ctrl+C
cancels from any prompt. Remove the Cancel rows.
<!-- NEXT_JS_LLM -->
