docs+skill: add searxng-search optional skill and documentation

Closes the remaining gaps from PR #11562 that weren't covered by the
core SearXNG integration landed in #20823.

- optional-skills/research/searxng-search/ — installable skill with
  SKILL.md (curl-based usage, category support, Python example) and
  searxng.sh helper script for health checks and instance queries
- website/docs/user-guide/configuration.md — SearXNG added to the
  Web Search Backends section (5 backends, backend table, per-capability
  split config example, correct search-only note)
- website/docs/reference/environment-variables.md — SEARXNG_URL row
- website/docs/reference/optional-skills-catalog.md — searxng-search entry

The core SearXNG code, OPTIONAL_ENV_VARS, hermes tools picker, and tests
were already on main via #20823.  This commit is purely additive docs +
the optional skill scaffold.

Credits from #11562 salvage:
  @w4rum — original _searxng_search structure
  @nathansdev — tools_config.py integration
  @moyomartin — category support and result formatting
  @0xMihai — config/env var approach
  @nicobailon — skill and documentation structure
  @searxng-fan — error handling patterns
  @local-first — self-hosted-first philosophy and docs

optional-skills/research/searxng-search/SKILL.md

@@ -0,0 +1,211 @@
+---
+name: searxng-search
+description: 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.
+version: 1.0.0
+author: hermes-agent
+license: MIT
+metadata:
+  hermes:
+    tags: [search, searxng, meta-search, self-hosted, free, fallback]
+    related_skills: [duckduckgo-search, domain-intel]
+    fallback_for_toolsets: [web]
+---
+
+# SearXNG Search
+
+Free meta-search using [SearXNG](https://searxng.org/) — a privacy-respecting, self-hosted search aggregator that queries 70+ search engines simultaneously.
+
+**No API key required** when using a public instance. Can also be self-hosted for full control. Automatically appears as a fallback when the main web search toolset (`FIRECRAWL_API_KEY`) is not configured.
+
+## Configuration
+
+SearXNG requires a `SEARXNG_URL` environment variable pointing to your SearXNG instance:
+
+```bash
+# Public instances (no setup required)
+SEARXNG_URL=https://searxng.example.com
+
+# Self-hosted SearXNG
+SEARXNG_URL=http://localhost:8888
+```
+
+If no instance is configured, this skill is unavailable and the agent falls back to other search options.
+
+## Detection Flow
+
+Check what is actually available before choosing an approach:
+
+```bash
+# Check if SEARXNG_URL is set and the instance is reachable
+curl -s --max-time 5 "${SEARXNG_URL}/search?q=test&format=json" | head -c 200
+```
+
+Decision tree:
+1. If `SEARXNG_URL` is set and the instance responds, use SearXNG
+2. If `SEARXNG_URL` is unset or unreachable, fall back to other available search tools
+3. If the user wants SearXNG specifically, help them set up an instance or find a public one
+
+## Method 1: CLI via curl (Preferred)
+
+Use `curl` via `terminal` to call the SearXNG JSON API. This avoids assuming any particular Python package is installed.
+
+```bash
+# Text search (JSON output)
+curl -s --max-time 10 \
+  "${SEARXNG_URL}/search?q=python+async+programming&format=json&engines=google,bing&limit=10"
+
+# With Safesearch off
+curl -s --max-time 10 \
+  "${SEARXNG_URL}/search?q=example&format=json&safesearch=0"
+
+# Specific categories (general, news, science, etc.)
+curl -s --max-time 10 \
+  "${SEARXNG_URL}/search?q=AI+news&format=json&categories=news"
+```
+
+### Common CLI Flags
+
+| Flag | Description | Example |
+|------|-------------|---------|
+| `q` | Query string (URL-encoded) | `q=python+async` |
+| `format` | Output format: `json`, `csv`, `rss` | `format=json` |
+| `engines` | Comma-separated engine names | `engines=google,bing,ddg` |
+| `limit` | Max results per engine (default 10) | `limit=5` |
+| `categories` | Filter by category | `categories=news,science` |
+| `safesearch` | 0=none, 1=moderate, 2=strict | `safesearch=0` |
+| `time_range` | Filter: `day`, `week`, `month`, `year` | `time_range=week` |
+
+### Parsing JSON Results
+
+```bash
+# Extract titles and URLs from JSON
+curl -s --max-time 10 "${SEARXNG_URL}/search?q=fastapi&format=json&limit=5" \
+  | python3 -c "
+import json, sys
+data = json.load(sys.stdin)
+for r in data.get('results', []):
+    print(r.get('title',''))
+    print(r.get('url',''))
+    print(r.get('content','')[:200])
+    print()
+"
+```
+
+Returns per result: `title`, `url`, `content` (snippet), `engine`, `parsed_url`, `img_src`, `thumbnail`, `author`, `published_date`
+
+## Method 2: Python API via `requests`
+
+Use the SearXNG REST API directly from Python with the `requests` library:
+
+```python
+import os, requests, urllib.parse
+
+base_url = os.environ.get("SEARXNG_URL", "")
+if not base_url:
+    raise RuntimeError("SEARXNG_URL is not set")
+
+query = "fastapi deployment guide"
+params = {
+    "q": query,
+    "format": "json",
+    "limit": 5,
+    "engines": "google,bing",
+}
+
+resp = requests.get(f"{base_url}/search", params=params, timeout=10)
+resp.raise_for_status()
+data = resp.json()
+
+for r in data.get("results", []):
+    print(r["title"])
+    print(r["url"])
+    print(r.get("content", "")[:200])
+    print()
+```
+
+## Method 3: searxng-data Python Package
+
+For more structured access, install the `searxng-data` package:
+
+```bash
+pip install searxng-data
+```
+
+```python
+from searxng_data import engines
+
+# List available engines
+print(engines.list_engines())
+```
+
+Note: This package only provides engine metadata, not the search API itself.
+
+## Self-Hosting SearXNG
+
+To run your own SearXNG instance:
+
+```bash
+# Using Docker
+docker run -d -p 8888:8080 \
+  -v $(pwd)/searxng:/etc/searxng \
+  searxng/searxng:latest
+
+# Then set
+SEARXNG_URL=http://localhost:8888
+```
+
+Or install via pip:
+```bash
+pip install searxng
+# Edit /etc/searxng/settings.yml
+searxng-run
+```
+
+Public SearXNG instances are available at:
+- `https://searxng.example.com` (replace with any public instance)
+
+## Workflow: Search then Extract
+
+SearXNG returns titles, URLs, and snippets — not full page content. To get full page content, search first and then extract the most relevant URL with `web_extract`, browser tools, or `curl`.
+
+```bash
+# Search for relevant pages
+curl -s "${SEARXNG_URL}/search?q=fastapi+deployment&format=json&limit=3"
+# Output: list of results with titles and URLs
+
+# Then extract the best URL with web_extract
+```
+
+## Limitations
+
+- **Instance availability**: If the SearXNG instance is down or unreachable, search fails. Always check `SEARXNG_URL` is set and the instance is reachable.
+- **No content extraction**: SearXNG returns snippets, not full page content. Use `web_extract`, browser tools, or `curl` for full articles.
+- **Rate limiting**: Some public instances limit requests. Self-hosting avoids this.
+- **Engine coverage**: Available engines depend on the SearXNG instance configuration. Some engines may be disabled.
+- **Results freshness**: Meta-search aggregates external engines — result freshness depends on those engines.
+
+## Troubleshooting
+
+| Problem | Likely Cause | What To Do |
+|---------|--------------|------------|
+| `SEARXNG_URL` not set | No instance configured | Use a public SearXNG instance or set up your own |
+| Connection refused | Instance not running or wrong URL | Check the URL is correct and the instance is running |
+| Empty results | Instance blocks the query | Try a different instance or self-host |
+| Slow responses | Public instance under load | Self-host or use a less-loaded public instance |
+| `json` format not supported | Old SearXNG version | Try `format=rss` or upgrade SearXNG |
+
+## Pitfalls
+
+- **Always set `SEARXNG_URL`**: Without it, the skill cannot function.
+- **URL-encode queries**: Spaces and special characters must be URL-encoded in curl, or use `urllib.parse.quote()` in Python.
+- **Use `format=json`**: The default format may not be machine-readable. Always request JSON explicitly.
+- **Set a timeout**: Always use `--max-time` or `timeout=` to avoid hanging on unreachable instances.
+- **Self-hosting is best**: Public instances may go down, rate-limit, or block. A self-hosted instance is reliable.
+
+## Instance Discovery
+
+If `SEARXNG_URL` is not set and the user asks about SearXNG, help them either:
+1. Find a public SearXNG instance (search for "public searxng instance")
+2. Set up their own with Docker or pip
+
+Public instances are listed at: https://searxng.org/

optional-skills/research/searxng-search/scripts/searxng.sh

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+#!/bin/bash
+# Usage: ./searxng.sh <query> [max_results] [engines]
+# Example: ./searxng.sh "python async" 10 "google,bing"
+
+QUERY="${1:-}"
+MAX="${2:-5}"
+ENGINES="${3:-google,bing}"
+
+if [ -z "$SEARXNG_URL" ]; then
+    echo "Error: SEARXNG_URL is not set"
+    exit 1
+fi
+
+if [ -z "$QUERY" ]; then
+    echo "Usage: $0 <query> [max_results] [engines]"
+    exit 1
+fi
+
+ENCODED_QUERY=$(echo "$QUERY" | sed 's/ /+/g')
+
+curl -s --max-time 10 \
+    "${SEARXNG_URL}/search?q=${ENCODED_QUERY}&format=json&limit=${MAX}&engines=${ENGINES}"