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/loader.ts

@@ -99,6 +99,7 @@ function createNoOpUIContext(): ExtensionUIContext {
 		setEditorText: () => {},
 		getEditorText: () => "",
 		editor: async () => undefined,
+		setEditorComponent: () => {},
 		get theme() {
 			return theme;
 		},