feat: 添加 UI 组件增强与测试覆盖

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/messages/UserForkBoilerplateMessage.tsx

@@ -0,0 +1,30 @@
+/**
+ * UserForkBoilerplateMessage — render the fork/subagent boilerplate directive.
+ */
+import type { TextBlockParam } from '@anthropic-ai/sdk/resources/index.mjs';
+import * as React from 'react';
+import { Box, Text } from '@anthropic/ink';
+import { extractTag } from '../../utils/messages.js';
+
+type Props = {
+  addMargin: boolean;
+  param: TextBlockParam;
+};
+
+export function UserForkBoilerplateMessage({ param, addMargin }: Props): React.ReactNode {
+  const text = param.text;
+  const extracted = extractTag(text, 'fork-boilerplate');
+  if (!extracted) {
+    return null;
+  }
+
+  const firstLine = extracted.trim().split('\n')[0] ?? '';
+  const preview = firstLine.length > 80 ? firstLine.slice(0, 77) + '...' : firstLine;
+
+  return (
+    <Box flexDirection="row" marginTop={addMargin ? 1 : 0}>
+      <Text dimColor>[fork] </Text>
+      <Text>{preview}</Text>
+    </Box>
+  );
+}