[direnv] Create module

nixos/modules/programs/direnv/default.nix

@@ -0,0 +1,22 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = sisyphus.programs.direnv;
+in {
+  options.sisyphus.programs.direnv.enable = lib.mkEnableOption "direnv";
+
+  config = lib.mkIf cfg.enable {
+    programs.direnv = {
+      enable = true;
+      nix-direnv.enable = true;  # Use nix-specific direnv.
+      persistDerivations = true; # Sets the extraOptions listed below.
+    };
+
+    # This is also done by setting programs.direnv.persistDerivations.
+    # Keep derivations so shells don't break.
+    nix.extraOption = ''
+      keep-outputs = true
+      keep-derivations = true
+    '';
+  };
+};