Add multi-provider web search support: Brave (default), Tavily,
DuckDuckGo, and SearXNG. Falls back to DuckDuckGo when provider
credentials are missing. Providers are dispatched via a map with
register_provider() for plugin extensibility.
- WebSearchConfig with env-var resolution and from_legacy() bridge
- Config migration for legacy flat keys (tavilyApiKey, searxngBaseUrl)
- SearXNG URL validation, explicit error for unknown providers
- ddgs package (replaces deprecated duckduckgo-search)
- 16 tests covering all providers, fallback, env resolution, edge cases
- docs/web-search.md with full config reference
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
