Salvage of PR #1707 by @kshitijk4poor (cherry-picked with authorship preserved).
Adds Tavily 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
- All three tools supported: search, extract, crawl
- TAVILY_API_KEY in config registry, doctor, status, setup wizard
- 15 new Tavily tests + 9 backend selection tests + 5 config tests
- Backward compatible
Closes#1707
