fix: 3 bugs found in Docker clean-room test

1. Install command: git+... requires git (many envs don't have it)
   → Changed to zip URL: pip install .../archive/main.zip

2. yt-dlp not in dependencies (shows warning in doctor)
   → Added yt-dlp to required dependencies

3. search-twitter/reddit shows raw 401 error without Exa key
   → Now shows friendly 'Exa API key not configured' message

pyproject.toml

@@ -26,6 +26,7 @@ dependencies = [
     "loguru>=0.7",
     "pyyaml>=6.0",
     "rich>=13.0",
+    "yt-dlp>=2024.0",
 ]
 
 [project.optional-dependencies]