Function expand_user_path 

pub fn expand_user_path(raw: &str) -> PathBuf

allow_paths / extra_roots come from config.toml, where no shell ever runs — users writing "$HOME/code" or "~/code" got a literal, never-matching prefix and concluded the whole option was broken (GH #392). Unset variables are left verbatim (and warned about) so the entry fails loudly in lean-ctx doctor instead of silently matching something else.