declarative codex install via npm activation

modules/user/packages.nix

@@ -71,5 +71,12 @@
     dconf
   ];
 
-
+  # ── Codex (npm, cutting-edge) ──────────────────────────
+  # Declarative: auto-installs/updates on every rebuild
+  home.activation.installCodex = config.lib.dag.entryAfter [ "writeBoundary" ] ''
+    export PATH="${pkgs.nodejs}/bin:$HOME/.npm-global/bin:$PATH"
+    mkdir -p "$HOME/.npm-global"
+    ${pkgs.nodejs}/bin/npm config set prefix "$HOME/.npm-global"
+    ${pkgs.nodejs}/bin/npm i -g @openai/codex 2>/dev/null || true
+  '';
 }