refactor(nixos): scope agent guidance

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

@@ -22,6 +22,17 @@ Do not call `lib.removeSuffix ".nix"` outside the utility definition.
 
 The utility centralizes Nix file-name normalization.
 
+## Qualify library functions
+
+### Rule
+
+Keep library functions qualified as `lib.<function>`.
+
+### Rationale
+
+Qualification makes library ownership explicit.
+It prevents local namespace collisions.
+
 ## Use executable helpers for package commands
 
 ### Rule