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

tests/hermes_cli/test_status.py

@@ -0,0 +1,14 @@
+from types import SimpleNamespace
+
+from hermes_cli.status import show_status
+
+
+def test_show_status_includes_tavily_key(monkeypatch, capsys, tmp_path):
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+    monkeypatch.setenv("TAVILY_API_KEY", "tvly-1234567890abcdef")
+
+    show_status(SimpleNamespace(all=False, deep=False))
+
+    output = capsys.readouterr().out
+    assert "Tavily" in output
+    assert "tvly...cdef" in output