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/SnipBoundaryMessage.tsx

@@ -0,0 +1,23 @@
+/**
+ * SnipBoundaryMessage — visual separator showing where conversation was snipped.
+ */
+import * as React from 'react';
+import { Box, Text } from '@anthropic/ink';
+import type { Message } from '../../types/message.js';
+
+type Props = {
+  message: Message;
+};
+
+export function SnipBoundaryMessage({ message }: Props): React.ReactNode {
+  const content =
+    typeof (message as Record<string, unknown>).content === 'string'
+      ? ((message as Record<string, unknown>).content as string)
+      : '[snip] Conversation history before this point has been snipped.';
+
+  return (
+    <Box marginTop={1} marginBottom={1}>
+      <Text dimColor>── {content} ──</Text>
+    </Box>
+  );
+}