dsh-tool-web-enhanced: Add a composable retrieval module for DeepSeek Harness's web_search

dsh-tool-web-enhanced is a DSH plugin maintained by edusrez, serving as a replacement for the native web_search. It aims to integrate multiple retrieval sources through a sectioned structure. The plugin prioritizes native search results while embedding optional additional modules, such as SearXNG, local Markdown RAG, and Parallel, into the same response. If no additional modules are configured, its behavior is consistent with the native tool; when modules are missing or unreachable, it performs silent degradation without causing the overall request to fail. Core functionalities include support for optional 'topic' and 'sources' parameters, as well as the 'parallel-extract' document extraction feature, which is disabled by default. Before installation, ensure dependencies such as @deepseek-ai/cordis are satisfied and check this.