feat(web): add SearXNG as a native search-only backend

Adds SearXNG as a free, self-hosted web search provider.  SearXNG is a
privacy-respecting metasearch engine that requires no API key — just a
running instance and SEARXNG_URL pointing at it.

## What this adds

- `tools/web_providers/searxng.py` — `SearXNGSearchProvider` implementing
  `WebSearchProvider` (search only; no extract capability)
- `_is_backend_available("searxng")` — gates on SEARXNG_URL
- `_get_backend()` — accepts "searxng" as a configured value; adds it to
  auto-detect candidates (lower priority than paid services)
- `web_search_tool` — dispatches to SearXNG when it is the active backend
- `check_web_api_key()` — includes SearXNG in availability check
- `OPTIONAL_ENV_VARS["SEARXNG_URL"]` — registered with tools=["web_search"]
- `tools_config.py` — SearXNG appears in the `hermes tools` provider picker
- `nous_subscription.py` — `direct_searxng` detection, web_active / web_available
- `setup.py` — SEARXNG_URL listed in the missing-credential hint
- 23 tests covering: is_configured, happy-path search, score sorting, limit,
  HTTP/request errors, _is_backend_available, _get_backend, check_web_api_key

## Config

```yaml
# Use SearXNG for search, any paid provider for extract
web:
  search_backend: "searxng"
  extract_backend: "firecrawl"

# Or: SearXNG as the sole backend (web_extract will use the next available)
web:
  backend: "searxng"
```

SearXNG is search-only — it does not implement WebExtractProvider.  Users
who only configure SEARXNG_URL get web_search available; web_extract falls
back to the next available extract provider (or is unavailable if none).

Closes #19198 (Phase 2 Task 4 — SearXNG provider)
Ref: #11562 (original SearXNG PR)

hermes_cli/tools_config.py

@@ -299,6 +299,15 @@ TOOL_CATEGORIES = {
                     {"key": "FIRECRAWL_API_URL", "prompt": "Your Firecrawl instance URL (e.g., http://localhost:3002)"},
                 ],
             },
+            {
+                "name": "SearXNG",
+                "badge": "free · self-hosted · search only",
+                "tag": "Privacy-respecting metasearch engine — search only (pair with any extract provider)",
+                "web_backend": "searxng",
+                "env_vars": [
+                    {"key": "SEARXNG_URL", "prompt": "Your SearXNG instance URL (e.g., http://localhost:8080)", "url": "https://searxng.github.io/searxng/"},
+                ],
+            },
         ],
     },
     "image_gen": {