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/config.py

@@ -1832,6 +1832,14 @@ OPTIONAL_ENV_VARS = {
         "password": True,
         "category": "tool",
     },
+    "SEARXNG_URL": {
+        "description": "URL of your SearXNG instance for free self-hosted web search",
+        "prompt": "SearXNG URL (e.g. http://localhost:8080)",
+        "url": "https://searxng.github.io/searxng/",
+        "tools": ["web_search"],
+        "password": False,
+        "category": "tool",
+    },
     "BROWSERBASE_API_KEY": {
         "description": "Browserbase API key for cloud browser (optional — local browser works without this)",
         "prompt": "Browserbase API key",