feat(flake): restructure nixvim input to include follows attribute

flake.nix

@@ -21,7 +21,10 @@
       url = "github:gmodena/nix-flatpak/?ref=latest";
     };
     vscode-server.url = "github:nix-community/nixos-vscode-server";
-    nixvim.url = "github:nix-community/nixvim";
+    nixvim = {
+      url = "github:nix-community/nixvim";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
     llm-agents.url = "github:numtide/llm-agents.nix";
     nix-vscode-extensions.url = "github:nix-community/nix-vscode-extensions";
     agenix = {