dsh-model-pin: Limit model request scope to prevent llama.cpp router reloads

The DeepSeek Harness (DSH) plugin dsh-model-pin resolves the “model roulette” problem, preventing local model reloads, cache loss, and routing errors caused by requests to disallowed models. The plugin enforces an allowlist at the resolution layer, forcing mismatched requests to be redirected or rejected, ensuring that sub-agents and background tasks always use the specified model. It is suitable for resource-constrained environments and scenarios requiring strict routing control, and can be easily enabled and customized via configuration files.