dsh-llamacpp: Local llama.cpp Adapter

`dsh-llamacpp` is an adapter plugin designed specifically for DeepSeek Harness (DSH) to connect to a local llama.cpp server. It communicates through an OpenAI-compatible interface, registers as the `llamacpp` provider in Harness, and supports streaming output and text-based tool calling. The plugin allows the server address, model name, and API key to be configured via environment variables or a configuration file. It currently supports text only, does not support images or Ollama, and includes robust error handling.