Add Google Gemini as AI provider

- Add gemini.py provider using google-genai SDK
- Update config.py with gemini provider and GEMINI_API_KEY
- Update ai_service.py factory to support gemini
- Add google-genai to requirements.txt
- Update .env.example, README.md, and CLAUDE.md documentation

src/daemon_boyfriend/services/providers/__init__.py

@@ -2,6 +2,7 @@
 
 from .anthropic import AnthropicProvider
 from .base import AIProvider, AIResponse, Message
+from .gemini import GeminiProvider
 from .openai import OpenAIProvider
 from .openrouter import OpenRouterProvider
 
@@ -12,4 +13,5 @@ __all__ = [
     "OpenAIProvider",
     "OpenRouterProvider",
     "AnthropicProvider",
+    "GeminiProvider",
 ]