dsh-fetch-third-party: Safe Third-Party Web Fetch for DeepSeek Harness

The `dsh-fetch-third-party` plugin addresses SSRF security risks in web scraping with DeepSeek Harness (DSH) through a “courier mode.” It delegates requests to user-configured third-party services instead of making direct local connections, thereby eliminating the attack surface. The plugin supports multiple providers, including Tavily and Jina Reader, as well as a custom Crawl4AI stack, and features secure credential storage, session-level budget limits, scraping caching, and structured output, ensuring safe and efficient retrieval of web content in DSH environments.