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/tui/src/index.ts

@@ -20,6 +20,8 @@ export { type SettingItem, SettingsList, type SettingsListTheme } from "./compon
 export { Spacer } from "./components/spacer.js";
 export { Text } from "./components/text.js";
 export { TruncatedText } from "./components/truncated-text.js";
+// Editor component interface (for custom editors)
+export type { EditorComponent } from "./editor-component.js";
 // Keybindings
 export {
 	DEFAULT_EDITOR_KEYBINDINGS,