docs: Add undo/redo and $EDITOR to UI navigation docs

- Added Alt+U/Alt+R for undo/redo in key bindings table
- Added tips about undo/redo (50 states) and $EDITOR button

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

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -298,11 +298,15 @@ vnoremap <leader>ec :!explain-code<CR>
 | **Select text** | `Shift` + mouse drag |
 | **Copy** | Terminal native (with Shift) |
 | **Paste** | `Ctrl+Shift+V` |
+| **Undo** (code editor) | `Alt+U` |
+| **Redo** (code editor) | `Alt+R` |
 
 **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.
+- The code editor supports undo/redo (up to 50 states) with `Alt+U` and `Alt+R`.
+- Use the `$EDITOR` button to open code or prompts in your external editor.
 
 ## AI-Assisted Code Generation