home/common/nix: install direnv

home/common/nix.nix

@@ -21,6 +21,14 @@ in {
     }
 
     (lib.mkIf standalone {
+      # direnv is a tool to automatically load shell environments upon entering
+      # a directory. nix-direnv has an extensionn to keep nix shells in the
+      # system's gcroots so shells can be used after a gc without rebuilding.
+      programs.direnv = {
+        enable = lib.mkDefault true;
+        nix-direnv.enable = lib.mkDefault true;
+      };
+
       home.sessionVariables.NIX_PATH = "nixpkgs=${nixpkgs}\${NIX_PATH:+:\${NIX_PATH}}";
       nix.registry = {
         nixpkgs = {