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

README.md

@@ -22,7 +22,7 @@ Your Agent reads the guide, installs everything, and configures itself. You just
 **Or install manually:**
 
 ```bash
-pip install git+https://github.com/Panniantong/agent-eyes.git
+pip install https://github.com/Panniantong/agent-eyes/archive/main.zip
 agent-eyes doctor    # See what's available
 agent-eyes setup     # Unlock more (optional)
 ```