Use XDG Base Directory defaults for Codex configuration

Summary

Codex currently defaults its user configuration directory to:

~/.codex/config.toml

On Linux, it should follow the XDG Base Directory Specification and default to:

${XDG_CONFIG_HOME:-$HOME/.config}/codex/config.toml

Rationale

Using the XDG default would:

• keep configuration files under the standard Linux configuration hierarchy;
• avoid adding another application-specific dot-directory directly under $HOME;
• improve consistency with other modern Linux CLI applications;
• make configuration management, backups, and dotfile tooling cleaner;
• respect users who already set XDG_CONFIG_HOME.

Suggested behavior

1. Use ${XDG_CONFIG_HOME}/codex/config.toml when XDG_CONFIG_HOME is set.
2. Otherwise use $HOME/.config/codex/config.toml.
3. Preserve CODEX_HOME as an explicit override for users who need a custom location.
4. Optionally retain backward compatibility by reading ~/.codex/config.toml when no XDG-path configuration exists, with a migration notice.

Environment

Linux / Arch Linux, but this applies generally to Linux and other XDG-aware Unix environments.

Reference: freedesktop.org XDG Base Directory Specification.