feat(direnv): add direnv with nix-direnv shell caching

hosts/neogaia/default.nix

@@ -47,6 +47,7 @@
   modules.fish.defaultShell = true;
 
   modules.git.enable = true;
+  modules.direnv.enable = true;
   modules.tmux.enable = true;
   modules.nvim.enable = true;
   modules.agents.claude-code.enable = true;

modules/direnv.nix

@@ -0,0 +1,24 @@
+{
+  config,
+  lib,
+  ...
+}:
+# direnv for the primary user, configured through home-manager.
+let
+  cfg = config.modules.direnv;
+  user = config.user.name;
+in
+{
+  options.modules.direnv.enable =
+    lib.mkEnableOption "direnv for the primary user, with nix-direnv shell caching";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.programs.direnv = {
+      enable = true;
+
+      # nix-direnv caches the evaluated shell so re-entering a directory is instant
+      # instead of re-running `nix develop`, and adds the `use flake` stdlib helper.
+      nix-direnv.enable = true;
+    };
+  };
+}