feat(coding-agent): add ctx.ui.setEditorComponent() extension API

- Add setEditorComponent() to ctx.ui for custom editor components
- Add CustomEditor base class for extensions (handles app keybindings)
- Add keybindings parameter to ctx.ui.custom() factory (breaking change)
- Add modal-editor.ts example (vim-like modes)
- Add rainbow-editor.ts example (animated text highlighting)
- Update docs: extensions.md, tui.md Pattern 7
- Clean up terminal on TUI render errors

packages/coding-agent/examples/extensions/README.md

@@ -45,6 +45,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 | `snake.ts` | Snake game with custom UI, keyboard handling, and session persistence |
 | `send-user-message.ts` | Demonstrates `pi.sendUserMessage()` for sending user messages from extensions |
 | `timed-confirm.ts` | Demonstrates AbortSignal for auto-dismissing `ctx.ui.confirm()` and `ctx.ui.select()` dialogs |
+| `modal-editor.ts` | Custom vim-like modal editor via `ctx.ui.setEditorComponent()` |
 
 ### Git Integration