dsh-a2a-agent: Expose DSH Agent as an A2A-compatible Agent

The `dsh-a2a-agent` is a component designed for the DeepSeek Harness (DSH) plugin environment, facilitating the conversion of DSH agents into A2A-compatible Agents. It provides Agent Card auto-discovery, JSON-RPC 2.0 endpoints, and LLM-driven conversational responses, supporting standard methods such as `message/send` and `tasks/get`. Available as an npm package or dynamic plugin, the component requires Node.js >= 20, reuses the host webServer, and supports CORS. When streaming configuration is enabled, it returns event streams via SSE; if the model fails, it automatically falls back to a deterministic echo. Data is stored in process memory only and is cleared upon termination. Use cases include integration with external A2A clients and unified discovery.