dsh-web-search-tavily: Switch DSH's web_search to Tavily, Usable Without an API Key

`dsh-web-search-tavily` is a DeepSeek Harness (DSH) community plugin maintained by cnChenKai, licensed under the MIT License. This plugin provides Tavily-based backend support for DSH's web search capabilities, enabling the built-in `web_search` tool to call the Tavily API instead of the default DeepSeek routing. Core features include support for Keyless mode, which accesses directly via the official Header when no API Key is configured, requiring no initial setup; if `TAVILY_API_KEY` is set (read from environment variables or a configuration file), it automatically switches to key-based mode to enhance quotas. Default parameters include basic search depth and 5