/add-dir Should be remembered in the current working directory

Like claude code  

The current working directory is the frontend project directory. After using `/add-dir` to add the backend project directory, you can develop both frontend and backend together.  

Currently, copilot cli's `/add-dir` is only valid for the current session, requiring re-addition every time, which is too cumbersome.  

Alternatively, let users choose whether to persist the memory.