feat(web): add Tavily as web search/extract/crawl backend (#1731)

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

hermes_cli/status.py

@@ -120,6 +120,7 @@ def show_status(args):
         "MiniMax": "MINIMAX_API_KEY",
         "MiniMax-CN": "MINIMAX_CN_API_KEY",
         "Firecrawl": "FIRECRAWL_API_KEY",
+        "Tavily": "TAVILY_API_KEY",
         "Browserbase": "BROWSERBASE_API_KEY",  # Optional — local browser works without this
         "FAL": "FAL_KEY",
         "Tinker": "TINKER_API_KEY",