codex: support XDG Base Directory specification

- Configuration file is now placed in XDG_CONFIG_HOME/codex/config.toml by default for versions >=0.2.0 when preferXdgDirectories is enabled.
- Falls back to ~/.codex/config.yaml for versions <0.2.0 and to ~/.codex/config.toml when preferXdgDirectories is disabled
- Sets CODEX_HOME environment variable to $XDG_CONFIG_HOME/codex when using XDG directories.
- Updated tests to verify XDG directory behavior and environment variable presence.

tests/modules/programs/codex/custom-instructions-prefer-xdg-directories.nix

@@ -0,0 +1,17 @@
+{
+  home.preferXdgDirectories = true;
+  programs.codex = {
+    enable = true;
+    custom-instructions = ''
+      - Always respond with emojis
+      - Only use git commands when explicitly requested
+    '';
+  };
+  nmt.script = ''
+    assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+      'export CODEX_HOME="/home/hm-user/.config/codex"'
+    assertFileExists home-files/.config/codex/AGENTS.md
+    assertFileContent home-files/.config/codex/AGENTS.md \
+      ${./AGENTS.md}
+  '';
+}

tests/modules/programs/codex/default.nix

@@ -1,7 +1,9 @@
 {
   codex-settings-toml = ./settings-toml.nix;
+  codex-settings-toml-prefer-xdg-directories = ./settings-toml-prefer-xdg-directories.nix;
   codex-settings-yaml = ./settings-yaml.nix;
   codex-empty-settings = ./empty-settings.nix;
   codex-custom-instructions = ./custom-instructions.nix;
+  codex-custom-instructions-prefer-xdg-directories = ./custom-instructions-prefer-xdg-directories.nix;
   codex-empty-custom-instructions = ./empty-custom-instructions.nix;
 }

tests/modules/programs/codex/empty-settings.nix

@@ -6,5 +6,6 @@
   nmt.script = ''
     assertPathNotExists home-files/.codex/config.toml
     assertPathNotExists home-files/.codex/config.yaml
+    assertFileNotRegex home-path/etc/profile.d/hm-session-vars.sh 'CODEX_HOME'
   '';
 }

tests/modules/programs/codex/settings-toml-prefer-xdg-directories.nix

@@ -0,0 +1,33 @@
+{ pkgs, ... }:
+let
+  codexPackage = pkgs.runCommand "codex-0.2.0" { } ''
+    mkdir -p $out/bin
+    echo '#!/bin/sh' > $out/bin/codex
+    chmod +x $out/bin/codex
+  '';
+in
+{
+  home.preferXdgDirectories = true;
+  programs.codex = {
+    enable = true;
+    package = codexPackage;
+    settings = {
+      model = "gemma3:latest";
+      model_provider = "ollama";
+      model_providers = {
+        ollama = {
+          name = "Ollama";
+          baseURL = "http://localhost:11434/v1";
+          envKey = "OLLAMA_API_KEY";
+        };
+      };
+    };
+  };
+  nmt.script = ''
+    assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+      'export CODEX_HOME="/home/hm-user/.config/codex"'
+    assertFileExists home-files/.config/codex/config.toml
+    assertFileContent home-files/.config/codex/config.toml \
+      ${./config.toml}
+  '';
+}

tests/modules/programs/codex/settings-toml.nix

@@ -26,5 +26,6 @@ in
     assertFileExists home-files/.codex/config.toml
     assertFileContent home-files/.codex/config.toml \
       ${./config.toml}
+    assertFileNotRegex home-path/etc/profile.d/hm-session-vars.sh 'CODEX_HOME'
   '';
 }