docs(nix-style): require executable helpers

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

@@ -21,3 +21,18 @@ Do not call `lib.removeSuffix ".nix"` outside the utility definition.
 ### Rationale
 
 The utility centralizes Nix file-name normalization.
+
+## Use executable helpers for package commands
+
+### Rule
+
+Use `lib.getExe` when invoking a package's main executable.
+Use `lib.getExe'` when invoking a non-main executable from a package.
+Do not build executable paths with `"${pkg}/bin/name"` when an executable helper can name the target.
+Use `lib.getExe' config.programs.<name>.package "tool"` when a command must follow a configured package option.
+
+### Rationale
+
+Executable helpers use the package output that contains binaries.
+They also make package ownership explicit.
+`lib.getExe'` avoids accidentally resolving a package's main executable when code needs a different tool.