XDG_CONFIG_HOME is not supported correctly

[ascarter]

Describe the bug

With XDG_CONFIG_HOME set to something like the standard $HOME/.config, copilot does not properly set its directory.

Affected version

GitHub Copilot CLI 0.0.406

Steps to reproduce the behavior

1. Set XDG_CONFIG_HOME=$HOME/.config
2. copilot
3. /mcp add
4. Enter a mcp server configuration
5. Outputs to $HOME/.config/.copilot
6. Should output to $HOME/.config/copilot

Expected behavior

The expected behavior with XDG_CONFIG_HOME is to not use a hidden directory. Instead, it should use a plain directory, clearly visible.

Copilot should be ~/.config/copilot

Additional context

Test on macOS and Linux.