## 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.