dsh-web-search-plugin: Unified web search interface for DeepSeek Harness

`dsh-web-search-plugin` is a unified web search plugin for DeepSeek Harness (DSH), designed to simplify the integration and management of multiple search engines. The plugin registers a unique `WebSearchProvider` (ID: `dsh-web-search`) via the `ctx.web` seam, integrates nine backends including DeepSeek, Tavily, and Brave Search, and supports switching engines directly in DSH settings without manually modifying underlying configuration. Core features include unified credential management, result normalization and deduplication, standardized error mapping, and quota progress display (e.g., for Tavily and Brave). Installation requires DSH version `0.1.2-alpha.4` or higher, using `p