add basic shell.nix

shell.nix

@@ -0,0 +1,65 @@
+{ pkgs ? import (builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/master.tar.gz";
+  }) {}
+}:
+
+pkgs.mkShell {
+  name = "codex-dev-shell";
+  dontDetectOcamlConflicts = true;
+  nativeBuildInputs = with pkgs.ocamlPackages; [
+    dune_3
+    findlib
+    # tests
+    mdx
+    mdx.bin
+    qcheck-core
+    # dev dependencies
+    merlin
+    ocaml
+    ocamlformat
+    ocp-browser
+    ocp-index
+    ocb
+    odoc
+  ];
+  buildInputs = with pkgs.ocamlPackages; [
+    js_of_ocaml
+    js_of_ocaml-ppx
+    ppx_deriving
+    ppx_inline_test
+  ];
+  propagatedBuildInputs = with pkgs.ocamlPackages; [
+    base64
+    bheap
+    camlp-streams
+    cudd
+    fmt
+    pacomb
+    patricia-tree
+    vdom
+    zarith
+  ];
+
+  shellHook = ''
+    mkdir -p utils/gui/deps/js
+
+    fetch_if_missing () {
+      if [ ! -f "$1" ]; then
+        echo "Fetching $1"
+        curl -L "$2" -o "$1"
+      fi
+    }
+
+    fetch_if_missing \
+      utils/gui/deps/tailwind4.1.5.css \
+      https://github.com/codex-semantics-library/codex/releases/download/1.0-rc4/tailwind4.1.5.css
+
+    fetch_if_missing \
+      utils/gui/deps/graphviz.umd.js \
+      https://github.com/codex-semantics-library/codex/releases/download/1.0-rc4/graphviz.umd.js
+
+    fetch_if_missing \
+      utils/gui/deps/js/bundle-output.js \
+      https://github.com/codex-semantics-library/codex/releases/download/1.0-rc4/bundle-output.js
+  '';
+}