Add DOS-style scrollbars and improve dialog navigation

- Add DOSScrollBar class with ▲/▼ arrow buttons for code/prompt editors
- Expand scrollbar click sensitivity (last 2 columns, zone-based scrolling)
- Fix urwid 3.x compatibility: use explicit ('pack', widget) tuples in Piles
- Add TabPassEdit class for proper Tab cycling in multiline editors
- Add TabCyclePile for focus cycling with Tab key in dialogs
- Add auto-adjust AI feature to automatically set output_var from code
- Update README with new UI navigation documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

README.md

@@ -291,11 +291,18 @@ vnoremap <leader>ec :!explain-code<CR>
 | Go back | `Escape` |
 | Select | `Enter` or click |
 | Navigate | Arrow keys |
+| **Scroll content** | Mouse wheel |
+| **Scroll up** | Click top of scrollbar |
+| **Scroll down** | Click bottom of scrollbar |
+| **Page up/down** | Click middle of scrollbar |
 | **Select text** | `Shift` + mouse drag |
 | **Copy** | Terminal native (with Shift) |
 | **Paste** | `Ctrl+Shift+V` |
 
-**Tip:** Hold `Shift` while using mouse for terminal-native text selection.
+**Tips:**
+- Hold `Shift` while using mouse for terminal-native text selection.
+- Code/Prompt editors have DOS-style scrollbars with `▲` and `▼` arrow buttons.
+- In step dialogs, use `Tab` to cycle between File, Editor, and Output fields.
 
 ## Philosophy