Add the LLM_* variables to .env.example
This issue requests the addition of four missing LLM-related environment variables (`LLM_BASE_URL`, `LLM_API_KEY`, `LLM_MODEL`, `LLM_PROXY`) to the `.env.example` file. These variables are crucial for users wanting to configure their own LLM providers but are currently undiscoverable for those who only use the example file for setup.