docs: add provider-agnostic llm environment configuration

Supporting Groq/Gemini requires explicit environment schema beyond a single OpenAI key. This file should define provider selection and per-provider keys/endpoints/models so users can start without guessing configuration.

Affected files: .env.example

Signed-off-by: ChinhLee <76194645+chinhkrb113@users.noreply.github.com>

.env.example

@@ -1,5 +1,29 @@
 # LLM API配置（支持 OpenAI SDK 格式的任意 LLM API）
-# 推荐使用阿里百炼平台qwen-plus模型：https://bailian.console.aliyun.com/
+# The engine uses a single unified OpenAI-compatible interface.
+# Set LLM_BASE_URL + LLM_API_KEY + LLM_MODEL_NAME for your chosen provider.
+#
+# --- Provider quick-reference ---
+#
+# Alibaba Qwen (default / recommended):
+#   LLM_BASE_URL=https://dashscope.aliyuncs.com/compatible-mode/v1
+#   LLM_API_KEY=<dashscope key>   # https://bailian.console.aliyun.com/
+#   LLM_MODEL_NAME=qwen-plus
+#
+# OpenAI:
+#   LLM_BASE_URL=https://api.openai.com/v1
+#   LLM_API_KEY=<openai key>      # https://platform.openai.com/api-keys
+#   LLM_MODEL_NAME=gpt-4o-mini
+#
+# Groq:
+#   LLM_BASE_URL=https://api.groq.com/openai/v1
+#   LLM_API_KEY=<groq key>        # https://console.groq.com/keys
+#   LLM_MODEL_NAME=llama-3.3-70b-versatile
+#
+# Google Gemini (OpenAI-compatible endpoint):
+#   LLM_BASE_URL=https://generativelanguage.googleapis.com/v1beta/openai
+#   LLM_API_KEY=<gemini key>      # https://aistudio.google.com/app/apikey
+#   LLM_MODEL_NAME=gemini-2.0-flash
+#
 # 注意消耗较大，可先进行小于40轮的模拟尝试
 LLM_API_KEY=your_api_key_here
 LLM_BASE_URL=https://dashscope.aliyuncs.com/compatible-mode/v1
@@ -11,6 +35,7 @@ ZEP_API_KEY=your_zep_api_key_here
 
 # ===== 加速 LLM 配置（可选）=====
 # 注意如果不使用加速配置，env文件中就不要出现下面的配置项
+# Accepts the same provider options listed above.
 LLM_BOOST_API_KEY=your_api_key_here
 LLM_BOOST_BASE_URL=your_base_url_here
-LLM_BOOST_MODEL_NAME=your_model_name_here
+LLM_BOOST_MODEL_NAME=your_model_name_here