Adds Tavily (tavily.com) as a third web backend alongside Firecrawl
and Parallel using the Tavily REST API via httpx.
- Backend selection via hermes tools → saved as web.backend in config.yaml
- Tavily support for all three tools: search, extract, and crawl
- TAVILY_API_KEY in config registry, doctor, status, setup wizard
- _tavily_request() helper with error handling
- Normalizer functions for search results and documents
- 15 new Tavily-specific tests + 9 backend selection tests
- Backward compatible — existing Firecrawl/Parallel users unaffected
Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>
