Adhere to the macOS File System Programming Guide on macOS

The user's home directory is not the place to dump program data, and for future cross-platform compatibility handling this would be inappropriate. Currently Codex stores user data in ~/.codex (modulated by a custom CODEX_HOME environment variable), however Apple has a specification for where to place files of various types (link). In Codex's case, ~/Library/Application Support/Codex would be suitable. (more info)