AskUserQuestion: preview panel should be scrollable

Problem

The preview field on AskUserQuestion options truncates content with a "N lines hidden" indicator when it exceeds the panel height. There's no way to configure the panel size or scroll to see the full content.

Screenshot

<img width="1262" height="429" alt="Image" src="https://github.com/user-attachments/assets/a5d7c539-c79d-494d-933f-dac805ebfc46" />

Example

When presenting commit split options, even a short preview like this gets truncated:

1. feat: Add terminal bell hooks for notifications
   Files: settings.json (hooks section), hooks/stop.py

2. fix: Use current context tokens in statusline
   Files: statusline.sh, settings.json (no overlap — hooks
   section is additive)

Suggested fix

Make the preview panel scrollable so users can see the full content, rather than hard-truncating it.