feat: add llms.txt downloader with error handling

tests/test_llms_txt_downloader.py

@@ -0,0 +1,12 @@
+import pytest
+from cli.llms_txt_downloader import LlmsTxtDownloader
+
+def test_download_llms_txt():
+    """Test downloading llms.txt content"""
+    downloader = LlmsTxtDownloader("https://hono.dev/llms-full.txt")
+
+    content = downloader.download()
+
+    assert content is not None
+    assert len(content) > 100  # Should have substantial content
+    assert isinstance(content, str)