feat: Add reusable indent utility function with tests

- Create indent function in src/lib/util.ts with options-based API
- Add comprehensive test suite in src/lib/util.spec.ts using bun:test
- Refactor MessageItem to use shared indent function for multi-line user messages
- Support skipFirstLine option for flexible indentation patterns

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: koki-develop

src/components/MessageItem.tsx

@@ -1,5 +1,6 @@
 import { Box, Text } from "ink";
 import type React from "react";
+import { indent } from "../lib/util";
 import type { Message } from "./types";
 
 interface MessageItemProps {
@@ -12,7 +13,7 @@ export const MessageItem: React.FC<MessageItemProps> = ({ message }) => {
   return (
     <Box marginBottom={1} paddingLeft={isUser ? 0 : 2}>
       <Text color={isUser ? "gray" : "cyan"}>
-        {isUser ? `> ${message.text}` : message.text}
+        {isUser ? `> ${indent(message.text)}` : message.text}
       </Text>
     </Box>
   );

src/lib/util.spec.ts

@@ -0,0 +1,40 @@
+import { describe, expect, test } from "bun:test";
+import { indent } from "./util";
+
+describe("indent", () => {
+  test("should indent all lines except first by default", () => {
+    const input = "first line\nsecond line\nthird line";
+    const expected = "first line\n  second line\n  third line";
+    expect(indent(input)).toBe(expected);
+  });
+
+  test("should indent all lines when skipFirstLine is false", () => {
+    const input = "first line\nsecond line\nthird line";
+    const expected = "  first line\n  second line\n  third line";
+    expect(indent(input, { skipFirstLine: false })).toBe(expected);
+  });
+
+  test("should handle single line text", () => {
+    const input = "single line";
+    const expected = "single line";
+    expect(indent(input)).toBe(expected);
+  });
+
+  test("should handle empty string", () => {
+    const input = "";
+    const expected = "";
+    expect(indent(input)).toBe(expected);
+  });
+
+  test("should handle empty lines", () => {
+    const input = "first line\n\nthird line";
+    const expected = "first line\n  \n  third line";
+    expect(indent(input)).toBe(expected);
+  });
+
+  test("should work with empty options", () => {
+    const input = "first line\nsecond line";
+    const expected = "first line\n  second line";
+    expect(indent(input, {})).toBe(expected);
+  });
+});

src/lib/util.ts

@@ -0,0 +1,17 @@
+interface IndentOptions {
+  skipFirstLine?: boolean;
+}
+
+export function indent(text: string, options: IndentOptions = {}): string {
+  const { skipFirstLine = true } = options;
+  const lines = text.split("\n");
+
+  return lines
+    .map((line, index) => {
+      if (skipFirstLine && index === 0) {
+        return line;
+      }
+      return `  ${line}`;
+    })
+    .join("\n");
+}