dsh-smart-router: DeepSeek Harness Smart Model Routing Plugin

The `dsh-smart-router` plugin implements intelligent routing via a virtual model, automatically switching to configured models based on request complexity and visual requirements without manual intervention. Core features include difficulty-based routing (hard, normal, and easy tiers), automatic image detection to invoke a vision model, and two classification modes: heuristic or LLM-based. Configuration requires specifying the model for each tier and the visual mode (replacement or pass-through), and it is recommended to use models from the same provider to optimize caching. The plugin supports a fail-open fallback mechanism and can coexist with existing vision plugins.