um-dsh-websearch: DeepSeek Harness Multi-Source Web Search Plugin

`um-dsh-websearch` is a web search plugin for DeepSeek Harness, designed to centrally manage multiple third-party data sources such as Exa, Parallel, and DeepSeek. Through strategy-layer encapsulation, the plugin supports multi-provider prioritization, paid/free dual tiers, multi-key polling, concurrent fan-out, and optimistic caching, ensuring search flexibility and security. Users can install it via pnpm and easily configure the master switch, concurrency mode, and credentials using the official settings card. The plugin requires a DeepSeek Harness version between 0.2.0-rc.1 and 0.3.0, automatically takes over default search after installation, and reverts automatically upon uninstallation.