fix(config): detect symlinked config paths as same file

On Fedora Atomic Desktops (Silverblue, Kinoite, etc.), /home is a symlink to /var/home. This caused a false positive duplicate config warning:

  Warning: Multiple config files found. Only the first one will be used.
    Using: /var/home/user/.config/glab-cli/config.yml
    Ignoring: /home/user/.config/glab-cli/config.yml

The paths resolve to the same file, but string comparison treated them as different.

Setting XDG_CONFIG_HOME does not help as a workaround because the legacy config path check uses os.UserHomeDir() independently from the XDG library. The two mechanisms can resolve home directory paths differently when symlinks are involved.

Fix this by using os.SameFile() to compare files at the inode/device level, which correctly identifies symlinked paths pointing to the same file.

Assisted-by: Claude Code