Add Codex plugin manifest

[internet-dot]

I opened this as a small metadata pass for the Codex plugin surface in the repo.

A couple of repo-specific details I checked first:

• Model Context Protocol Marketplace
• 火山引擎大模型生态广场 目前已上线 100+ MCP Server，集成丰富的火山引擎官方云服务及优质三方生态工具。同时支持用户结合火山方舟大模型服务，快速跳转至火山方舟或其他支持 MCP 协议的平台（如 Trae、Cursor、Python 等），助力企业开发者精准打造符合自身业务场景的 AI 大模型应用，打通大模型应用落地的 “最后一公里”。
• 复制 URL 或 JSON，前往支持的MCP Client中进行安装与使用 MCP Server。

This PR adds the Codex plugin metadata, an .mcp.json starting point, and the scanner workflow.
If you want different command wiring or manifest metadata, I can adjust the branch.

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