config folder is out of place on Linux

Hi there!
(thank you for the great work and articles on coding agents, very inspiring)

I'm using pi on Linux, and noticed that the config folder is located directly within the user's $HOME directory.
On Linux, modern tools should follow the XDG Base Directory Spec to avoid cluttering the home root.

The TL;DR of that spec is basically:

To generate the config directory prefix, check for PI_CONFIG_DIR first, then XDG_CONFIG_HOME. If neither is set, use the OS default:

• Windows: %APPDATA%
• macOS: $HOME
• Linux/BSD: $HOME/.config

The full config directory path would then be <prefix>/pi.