docs(README): 更新WORKING_DIR字段的使用说明

在独立模式和路由模式下，WORKING_DIR字段的使用方式不同。添加注释说明该字段仅在独立模式下生效，而在路由模式下会被忽略。

README.md

@@ -140,6 +140,8 @@ OPENAI_API_KEY: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 OPENAI_MODEL: glm-4.7
 
 # Root directory for all project sessions (absolute path)
+# Only used in standalone mode (python standalone.py)
+# In router mode (python main.py), this field is ignored
 WORKING_DIR: C:/Users/yourname/projects
 
 # Allowlist of Feishu open_ids that may use the bot.