docs: Add Codex CLI configuration guide

[vincent067]

Summary

This PR adds documentation for using the Terraform MCP Server with Codex CLI, OpenAI's official coding agent.

Changes

Added a new section "Usage with Codex CLI" to the README.md that includes:

• Brief introduction to Codex CLI
• Installation prerequisites (Node.js 18+, OpenAI API key)
• Configuration examples for both stdio and HTTP transport modes
• Example commands showing how to add the MCP server to Codex
• JSON configuration examples consistent with other client sections (VS Code, Cursor, Claude Desktop, etc.)

Motivation

Codex CLI is gaining popularity as a terminal-based coding agent, and users need clear documentation on how to integrate the Terraform MCP Server with it. This documentation follows the same style and structure as existing sections for other MCP clients.

Testing

• Verified the markdown renders correctly
• Commands are based on Codex CLI's documented MCP configuration options

[github-actions]

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