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

.env.example

@@ -7,7 +7,7 @@ DISCORD_TOKEN=your_discord_bot_token_here
 # ===========================================
 # AI Provider Configuration
 # ===========================================
-# Available providers: "openai", "openrouter", "anthropic"
+# Available providers: "openai", "openrouter", "anthropic", "gemini"
 AI_PROVIDER=openai
 
 # Model to use (e.g., gpt-4o, gpt-4o-mini, claude-3-5-sonnet, etc.)
@@ -17,6 +17,7 @@ AI_MODEL=gpt-4o
 OPENAI_API_KEY=sk-xxx
 OPENROUTER_API_KEY=sk-or-xxx
 ANTHROPIC_API_KEY=sk-ant-xxx
+GEMINI_API_KEY=xxx
 
 # Maximum tokens in AI response (100-4096)
 AI_MAX_TOKENS=1024