refactor(dotfiles): centralize Nix file names

src/skills/style/nix/rules/general.md

@@ -10,3 +10,14 @@ Pass `util` to Nix functions that need the helper.
 
 `util.collectNixFiles` centralizes recursive discovery of Nix files.
 Repository Nix code uses one implementation.
+
+## Remove Nix file suffixes with the utility
+
+### Rule
+
+Use `util.removeNixSuffix` to remove a `.nix` suffix from a path.
+Do not call `builtins.baseNameOf` or `lib.removeSuffix ".nix"` outside the utility definition.
+
+### Rationale
+
+The utility centralizes Nix file-name normalization.