feat: add multi-provider LLM support with LiteLLM

- Replace openai with litellm for unified LLM interface
- Support 100+ providers: OpenAI, Anthropic, Gemini, Azure, Ollama, etc.
- Add custom API base URL support (LLM_API_BASE)
- Add .env file support with python-dotenv
- Add .env.example configuration template

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -24,6 +24,11 @@ ENV/
 *.sqlite
 *.sqlite3
 
+# Environment variables
+.env
+.env.local
+.env.*.local
+
 # Logs
 *.log