relay-dsh-plugin-codex: Using Codex in DeepSeek Harness

`relay-dsh-plugin-codex` is a plugin that provides a native Codex backend for DeepSeek Harness (DSH). It runs through the Codex App Server, allowing users to invoke Codex models seamlessly while maintaining a smooth DSH interface, and preserves DSH's session management, workspace, editor, and tooling capabilities. Installation requires Node.js 22.13 and pnpm, and users must log in via the Codex client before use. The plugin is currently in developer preview, with known issues including feature discrepancies and occasional empty local command events.