feat: add SearXNG as a native web search backend

Adds SearXNG (https://docs.searxng.org) as a self-hosted, privacy-first
web search backend alongside Firecrawl, Tavily, Exa, and Parallel.

SearXNG is a meta-search engine that aggregates results from 70+ search
engines. No API key needed -- just set SEARXNG_URL to your instance.

Changes:
- tools/web_tools.py: _get_searxng_url(), _searxng_search(), search
  dispatch, extract falls back to Firecrawl (SearXNG is search-only)
- hermes_cli/tools_config.py: SearXNG provider in web tool picker
- hermes_cli/config.py: SEARXNG_URL env var, diagnostics, set command
- tests/tools/test_web_tools_searxng.py: 15 tests
- optional-skills/research/searxng-search/: agent-guided skill
- Docs: configuration.md, environment-variables.md, skills catalogs

Based on #6071 by @gnanam1990, #8106 by @cro, #2572 by @bhovig,
#2710 and #9961 by @StreamOfRon, #7258 by @coldxiangyu163

hermes_cli/tools_config.py

@@ -235,6 +235,15 @@ TOOL_CATEGORIES = {
                     {"key": "TAVILY_API_KEY", "prompt": "Tavily API key", "url": "https://app.tavily.com/home"},
                 ],
             },
+            {
+                "name": "SearXNG",
+                "badge": "free · self-hosted",
+                "tag": "Open-source federated search (self-hosted)",
+                "web_backend": "searxng",
+                "env_vars": [
+                    {"key": "SEARXNG_URL", "prompt": "SearXNG instance URL (e.g., https://searx.example.com)"},
+                ],
+            },
             {
                 "name": "Firecrawl Self-Hosted",
                 "badge": "free · self-hosted",