dsh-smart-route: DeepSeek Harness Smart Routing Plugin

The DeepSeek Harness (DSH) plugin `dsh-smart-route` provides an intelligent vendor routing solution as a replacement for `polyglot`. It supports multi-channel chained routing, automatic fallback for all error codes, and an exponential backoff cooldown mechanism to ensure high availability. The plugin registers a single virtual provider with a clean interface, and supports one-click start/stop, retry policy configuration, and multi-chain management. After installation, it defaults to the `deepseek-official` chain, making it suitable for users who seek high availability and fine-grained control.