dsh-web-search-firecrawl: Connect DSH's web_search to Firecrawl

`@elves-ai/dsh-web-search-firecrawl` is a plugin for DeepSeek Harness (DSH) designed to switch the backend of the built-in `web_search` from the DeepSeek router to the Firecrawl search API. The plugin implements its functionality by registering a `WebSearchProvider`, enabling online switching between Firecrawl and the original route without requiring a restart. Before use, ensure the DSH version is `0.1.0-rc.6` and you possess a Firecrawl API key. If the key is not configured and there is no environment variable fallback, the search will fail due to `WEB_PROVIDER_CONFIGURED_UNAVAILABLE`.