dsh-openai-gateway: Expose DSH Agent as an OpenAI Compatible API

`dsh-openai-gateway` is a plugin for DSH designed to expose the DeepSeek Harness (DSH) Agent as a standard OpenAI-compatible API server. The plugin supports endpoints such as `POST /v1/chat/completions` and `GET /v1/models`; each request is backed by a real DSH Agent session equipped with tools and a workspace. Users simply need to install the plugin and restart the service to connect using any OpenAI client by configuring the `base_url` and API key. By default, the plugin listens on the local loopback address port 41540 and offers two session modes: stateless and sticky. Sticky sessions support multi-turn continuous tasks and require specific headers or fields to maintain the session state.