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/src/core/extensions/runner.ts

@@ -74,6 +74,7 @@ const noOpUIContext: ExtensionUIContext = {
 	setEditorText: () => {},
 	getEditorText: () => "",
 	editor: async () => undefined,
+	setEditorComponent: () => {},
 	get theme() {
 		return theme;
 	},