fix(hands): 集成 SearXNG 元搜索引擎 — 替换不可用的 DuckDuckGo Instant Answer API

- ResearcherHand 新增 search_searxng() 方法，调用 SearXNG JSON API 聚合 70+ 搜索引擎
- SearchEngine 枚举增加 SearXNG 变体，路由逻辑按配置分发搜索后端
- Auto 模式: SearXNG 优先 → DuckDuckGo fallback
- config.toml [tools.web.search] 新增 searxng_url/searxng_timeout 配置
- docker-compose.yml 新增 SearXNG 服务容器 (searxng-config/settings.yml)
- 新增 6 个 SearXNG 相关单元测试 (响应解析/URL构造/分数归一化/配置加载)
- 验证: 124 tests PASS, workspace 0 warnings

config/config.toml

@@ -223,8 +223,10 @@ timeout = "30s"
 [tools.web]
 [tools.web.search]
 enabled = true
-default_engine = "duckduckgo"
+default_engine = "searxng"
 max_results = 10
+searxng_url = "http://localhost:8888"
+searxng_timeout = 15
 
 # File system tool
 [tools.fs]