dsh-web-search-custom: Custom Search API Adapter

The `dsh-web-search-custom` plugin for the `web` profile in DeepSeek Harness (DSH) allows users to customize the search interface as an alternative to the default hosted backend, making it particularly suitable for self-hosted search stacks or local network environments. As a standard bundle, it is non-intrusive and dependency-free, and supports generic JSON APIs (`GET`/`POST`) as well as visual configuration. Key features include custom URLs, request methods, headers, timeouts, and field mapping, with built-in SearXNG compatibility. It can be configured via the settings page, a Profile Patch, or environment variables. Usage requires compliance with specific version requirements and supports result deduplication and format cleaning.