feat: add SearXNG as primary web search backend

[Nullislander0]

Summary

• Adds SearXNG (self-hosted meta-search engine) as the preferred web search backend
• Three-tier fallback: SearXNG → Brave Search API → DuckDuckGo Instant Answers
• SearXNG aggregates results from multiple engines (Google, Bing, DDG, Brave, etc.) and returns engine attribution and relevance scores
• Configured via SEARXNG_URL env var (defaults to http://localhost:8888)
• Bumps max results from 10 to 20
• Gracefully falls back to existing Brave/DDG path if SearXNG is unavailable (15s timeout)

This keeps the existing search behavior fully intact for users without SearXNG — the only change for them is the higher max results cap.

Test plan

• cargo build -p claurst-tools compiles clean
• Full workspace build passes on Windows 10
• Test with SearXNG running locally (SEARXNG_URL=http://localhost:8888)
• Test fallback when SearXNG is down (should fall through to Brave/DDG as before)
• Test with BRAVE_SEARCH_API_KEY set (second tier)
• Test with no env vars set (DDG fallback, existing behavior)

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