plugins/codex: add module

Closes #4220

plugins/by-name/codex/default.nix

@@ -0,0 +1,20 @@
+{ lib, ... }:
+lib.nixvim.plugins.mkNeovimPlugin {
+  name = "codex";
+  package = "codex-nvim";
+  description = "A Copilot-style coding assistant integration for Neovim.";
+
+  maintainers = [ lib.maintainers.khaneliman ];
+
+  dependencies = [
+    "codex"
+  ];
+
+  settingsExample = {
+    keymaps = {
+      toggle = "<leader>ac";
+    };
+    border = "rounded";
+    model = "gpt-5-codex";
+  };
+}

tests/test-sources/plugins/by-name/codex/default.nix

@@ -0,0 +1,35 @@
+{ lib }:
+{
+  empty = {
+    plugins.codex.enable = true;
+  };
+
+  defaults = {
+    plugins.codex = {
+      enable = true;
+      settings = {
+        keymaps = {
+          toggle = lib.nixvim.mkRaw "nil";
+          quit = "<C-q>";
+        };
+        border = "single";
+        width = 0.8;
+        height = 0.8;
+        cmd = "codex";
+        model = lib.nixvim.mkRaw "nil";
+        autoinstall = true;
+      };
+    };
+  };
+
+  example = {
+    plugins.codex = {
+      enable = true;
+      settings = {
+        keymaps.toggle = "<leader>ac";
+        border = "rounded";
+        model = "gpt-5-codex";
+      };
+    };
+  };
+}