Config: Add support for --add-dir in config.toml

What variant of Codex are you using?

CLI

What feature would you like to see?

Add a setting to config.toml, so whenever codex is invoked in a directory, it also includes configured directory to which codex (and sandbox) will have access to.

This is useful when CLIs (typically build tools for cache) actually output in a central directory outside the repository root and not everyone in the team might be aware, so we want those changes to persist in config.toml, so it automatically gets applied when someone invoked codex.

Additional information

_No response_