feat(coding-agent): make skill invocation messages collapsible

- Add ParsedSkillBlock interface and parseSkillBlock() function
- Change skill expansion to use XML-style <skill> tags
- Add SkillInvocationMessageComponent for collapsible display
- Collapsed: single line with skill name and expand hint
- User message rendered separately after skill block

Fixes #894

packages/coding-agent/src/modes/interactive/components/index.ts

@@ -21,6 +21,7 @@ export { type ModelsCallbacks, type ModelsConfig, ScopedModelsSelectorComponent
 export { SessionSelectorComponent } from "./session-selector.js";
 export { type SettingsCallbacks, type SettingsConfig, SettingsSelectorComponent } from "./settings-selector.js";
 export { ShowImagesSelectorComponent } from "./show-images-selector.js";
+export { SkillInvocationMessageComponent } from "./skill-invocation-message.js";
 export { ThemeSelectorComponent } from "./theme-selector.js";
 export { ThinkingSelectorComponent } from "./thinking-selector.js";
 export { ToolExecutionComponent, type ToolExecutionOptions } from "./tool-execution.js";