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

website/docs/reference/environment-variables.md

@@ -90,6 +90,7 @@ For native Anthropic auth, Hermes prefers Claude Code's own credential files whe
 | `FIRECRAWL_API_KEY` | Web scraping and cloud browser ([firecrawl.dev](https://firecrawl.dev/)) |
 | `FIRECRAWL_API_URL` | Custom Firecrawl API endpoint for self-hosted instances (optional) |
 | `TAVILY_API_KEY` | Tavily API key for AI-native web search, extract, and crawl ([app.tavily.com](https://app.tavily.com/home)) |
+| `SEARXNG_URL` | SearXNG instance URL for self-hosted federated web search ([docs.searxng.org](https://docs.searxng.org)) |
 | `EXA_API_KEY` | Exa API key for AI-native web search and contents ([exa.ai](https://exa.ai/)) |
 | `BROWSERBASE_API_KEY` | Browser automation ([browserbase.com](https://browserbase.com/)) |
 | `BROWSERBASE_PROJECT_ID` | Browserbase project ID |