feat(style-nix): require shared file discovery

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

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+## Discover Nix files
+
+### Rule
+
+Use `util.collectNixFiles` to discover Nix files.
+Do not use `builtins.readDir` with a `.nix` suffix filter for this purpose.
+Pass `util` to Nix functions that need the helper.
+
+### Rationale
+
+`util.collectNixFiles` centralizes recursive discovery of Nix files.
+Repository Nix code uses one implementation.