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/optional-skills-catalog.md

@@ -128,6 +128,7 @@ The largest optional category — covers the full ML pipeline from data curation
 | **bioinformatics** | Gateway to 400+ bioinformatics skills from bioSkills and ClawBio. Covers genomics, transcriptomics, single-cell, variant calling, pharmacogenomics, metagenomics, and structural biology. |
 | **domain-intel** | Passive domain reconnaissance using Python stdlib. Subdomain discovery, SSL certificate inspection, WHOIS lookups, DNS records, and bulk multi-domain analysis. No API keys required. |
 | **duckduckgo-search** | Free web search via DuckDuckGo — text, news, images, videos. No API key needed. |
+| **searxng-search** | Free meta-search via SearXNG — aggregates results from 70+ search engines. Self-hosted or use a public instance. No API key needed. |
 | **gitnexus-explorer** | Index a codebase with GitNexus and serve an interactive knowledge graph via web UI and Cloudflare tunnel. |
 | **parallel-cli** | Vendor skill for Parallel CLI — agent-native web search, extraction, deep research, enrichment, and monitoring. |
 | **qmd** | Search personal knowledge bases, notes, docs, and meeting transcripts locally using qmd — a hybrid retrieval engine with BM25, vector search, and LLM reranking. |