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/test/compaction-extensions.test.ts

@@ -137,6 +137,7 @@ describe.skipIf(!API_KEY)("Compaction extensions", () => {
 				setEditorText: () => {},
 				getEditorText: () => "",
 				editor: async () => undefined,
+				setEditorComponent: () => {},
 				get theme() {
 					return theme;
 				},