feat(web): configurable web search providers with fallback

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>

pyproject.toml

@@ -24,6 +24,7 @@ dependencies = [
     "websockets>=16.0,<17.0",
     "websocket-client>=1.9.0,<2.0.0",
     "httpx>=0.28.0,<1.0.0",
+    "ddgs>=9.5.5,<10.0.0",
     "oauth-cli-kit>=0.1.3,<1.0.0",
     "loguru>=0.7.3,<1.0.0",
     "readability-lxml>=0.8.4,<1.0.0",