dsh-webfetch: Adding read-only web fetching capability to DSH

dsh-webfetch is a read-only web fetching plugin for DeepSeek Harness (DSH) that supports running with zero dependencies in a Node >= 20 environment. The plugin provides four core tools: `web_fetch` for fetching http/https pages and extracting Markdown or plain text bodies; `web_links` for collecting and deduplicating absolute links on a page; `web_feed` for parsing structured entries from RSS 2.0 / Atom feeds; and `web_headers` for checking HTTP status codes, response headers, and redirect chains without downloading the body content. In terms of security and limitations, the plugin only supports http/https protocols, rejects file and ftp addresses, and does not send credentials.