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/skills-catalog.md

@@ -256,6 +256,7 @@ Skills for academic research, paper discovery, literature review, domain reconna
 | `llm-wiki` | Karpathy's LLM Wiki — build and maintain a persistent, interlinked markdown knowledge base. Ingest sources, query compiled knowledge, and lint for consistency. Unlike RAG, the wiki compiles knowledge once and keeps it current. Works as an Obsidian vault. Wiki path is controlled by the `WIKI_PATH` env var (defaults to `~/wiki`). | `research/llm-wiki` |
 | `domain-intel` | Passive domain reconnaissance using Python stdlib. Subdomain discovery, SSL certificate inspection, WHOIS lookups, DNS records, domain availability checks, and bulk multi-domain analysis. No API keys required. | `research/domain-intel` |
 | `duckduckgo-search` | Free web search via DuckDuckGo — text, news, images, videos. No API key needed. Prefer the `ddgs` CLI when installed; use the Python DDGS library only after verifying that `ddgs` is available in the current runtime. | `research/duckduckgo-search` |
+| `searxng-search` | Free meta-search via SearXNG — aggregates results from 70+ search engines. Self-hosted or use a public instance. No API key needed. Falls back automatically when the web search toolset is unavailable. | `research/searxng-search` |
 | `ml-paper-writing` | Write publication-ready ML/AI papers for NeurIPS, ICML, ICLR, ACL, AAAI, COLM. Use when drafting papers from research repos, structuring arguments, verifying citations, or preparing camera-ready submissions. Includes LaTeX templates, reviewer guidelines, and citation verificatio… | `research/ml-paper-writing` |
 | `polymarket` | Query Polymarket prediction market data — search markets, get prices, orderbooks, and price history. Read-only via public REST APIs, no API key needed. | `research/polymarket` |