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/README.md

@@ -355,6 +355,7 @@ Both modes are configurable via `/settings`: "one-at-a-time" delivers messages o
 | Ctrl+K | Delete to end of line |
 | Ctrl+Y | Paste most recently deleted text |
 | Alt+Y | Cycle through deleted text after pasting |
+| Ctrl+- | Undo |
 
 **Other:**
 
@@ -405,6 +406,7 @@ All keyboard shortcuts can be customized via `~/.pi/agent/keybindings.json`. Eac
 | `deleteToLineEnd` | `ctrl+k` | Delete to line end |
 | `yank` | `ctrl+y` | Paste most recently deleted text |
 | `yankPop` | `alt+y` | Cycle through deleted text after pasting |
+| `undo` | `ctrl+-` | Undo last edit |
 | `newLine` | `shift+enter` | Insert new line |
 | `submit` | `enter` | Submit input |
 | `tab` | `tab` | Tab/autocomplete |

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 |