feat(tui): add undo support to Editor with the Ctrl+- hotkey

Undo snapshots are captured for all edit operations:

- Word insertion, backspace, forward delete
- Word/line deletion (Ctrl+W, Ctrl+U, Ctrl+K, Alt+D)
- Yank/yank-pop, paste, autocomplete completion
- Cursor movement starts a new undo unit
- setText() pushes snapshot when content changes

Additionally, history browsing captures the undo state on first entry.

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -3446,6 +3446,7 @@ export class InteractiveMode {
 		const deleteToLineEnd = this.getEditorKeyDisplay("deleteToLineEnd");
 		const yank = this.getEditorKeyDisplay("yank");
 		const yankPop = this.getEditorKeyDisplay("yankPop");
+		const undo = this.getEditorKeyDisplay("undo");
 		const tab = this.getEditorKeyDisplay("tab");
 
 		// App keybindings
@@ -3481,6 +3482,7 @@ export class InteractiveMode {
 | \`${deleteToLineEnd}\` | Delete to end of line |
 | \`${yank}\` | Paste the most-recently-deleted text |
 | \`${yankPop}\` | Cycle through the deleted text after pasting |
+| \`${undo}\` | Undo |
 
 **Other**
 | Key | Action |