docs(nix-style): add package command rules

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

@@ -21,3 +21,32 @@ 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` for a package's main executable.
+Use `lib.getExe'` for a package's non-main executable.
+Do not build executable paths with `"${pkg}/bin/name"` when an executable helper names the target.
+Use `lib.getExe' config.programs.<name>.package "tool"` when a command follows a configured package option.
+
+### Rationale
+
+Executable helpers select the package output that contains binaries.
+They make package ownership explicit.
+`lib.getExe'` prevents calls to a package's main executable when code needs another tool.
+
+## Prefer NixOS package ownership for commands
+
+### Rule
+
+Use `pkgs.<name>` for Home Manager tools.
+Use `config.programs.<name>.package` when a NixOS module owns the package selection.
+Do not read `config.home-manager.users.<user>.programs.<name>.package` for command paths.
+
+### Rationale
+
+Home Manager uses the NixOS package set in this repository.
+`pkgs.<name>` avoids dependency paths through the user tree under Home Manager.
+Configured NixOS package options preserve package selection when a module owns the tool.