dsh-api-gateway: Expose DeepSeek Harness as REST + SSE interface

dsh-api-gateway is an MIT-licensed plugin for DeepSeek Harness (DSH), maintained by litestartup-com. It is designed to expose running DSH instances as HTTP APIs, providing stable REST + SSE interfaces for third-party scripts, command-line tools, and browser-side services. This plugin is not an independent inference service but rather an access layer for existing DSH sessions, supporting external clients in creating sessions, sending messages, and receiving streaming responses. Core features include providing 10 REST endpoints that support API key authentication, key rotation, and GUI state control; implementing token-level SSE streaming output; and strictly distinguishing between visible answers (text) and model reasoning content.