feat: support for dedicated configuration file for API keys

Current Behavior

Currently, the codex project supports setting the OpenAI API key in two ways:

1. As an environment variable: export OPENAI_API_KEY="your-api-key-here"
2. In a .env file at the root of the project: OPENAI_API_KEY=your-api-key-here

Requested Feature

Add support for storing the OpenAI API key in a dedicated, user-wide configuration file (e.g., ~/.codex.config) that would be specific to the codex tool.

Motivation

• Persistence: Avoids having to re-export environment variables in every new terminal session
• Separation of concerns: Keeps API keys separate from project files and specific to the codex tool
• Security: Provides a dedicated location for sensitive credentials outside of project directories
• User convenience: Makes it easier for users who work with multiple projects but want a consistent API key setup

Proposed Implementation

Add the ability to read OpenAI API keys from a dedicated configuration file at ~/.codex.config (or similar location). For example:

# ~/.codex.config
OPENAI_API_KEY=your-api-key-here

The configuration file could use a simple format like the existing .env approach or a more structured format like YAML/JSON if additional configuration options might be needed in the future.