docs: initialize NixOS infrastructure with AI assistant

Creates PROJECT.md with vision and requirements.
Creates config.json with interactive workflow mode.

users/n8n-worker.nix

@@ -0,0 +1,11 @@
+{ pkgs, inputs, config, keys, ... }: {
+  users.users.n8n-worker = {
+    isSystemUser = true;
+    group = "n8n-worker";
+    extraGroups = [ "docker" ];
+    shell = pkgs.bashInteractive;
+    openssh.authorizedKeys.keys = [
+      keys.users.n8n-worker.main
+    ];
+  };
+  users.groups.n8n-worker = {};