feat(search): web-search 接线本机 SearXNG 兜底 + 显式开关（她不再拿一把用不了的锤子）

主人报的故障：她调 web-search 必失败（no search endpoint configured）。根因两条：组合根从未注入 WithSearchEndpoint + 配置层根本没有搜索配置项。
- internal/config/websearch.go：WEB_SEARCH_ENDPOINT（默认本机兜底 http://127.0.0.1:38471）+ WEB_SEARCH_TIMEOUT；ResolveEndpoint() 三态裁决（可用/显式关闭/非法），非法值启动即报错
- cmd/cyrene/wire_io_tools.go：组合根唯一的 io 选项装配点；端点不可用 ⇒ WithoutSearch() + 带 source 的 WARN（只记键名与原因，不打印 URL 值）
- plugins/tools/io：新增 WithoutSearch()（**根本不进注册面** ⇒ 模型看不见）+ WithSearchTimeout()；协议解析逻辑一字未动
- 隐蔽坑（额外查出并修）：SearXNG 对 /?q= 回 308 → /search，重定向那一跳走**共享 SSRF 白名单** ⇒ 只注入端点照样被拦。改为**只给 web-search 一件**加宽自己的客户端与白名单，共享白名单与 http/web-fetch 客户端一字未动（有测试钉住它仍拒绝本机网关地址）
- .env.example 新增 2 键 + 注释说明「空串/off/none = 不注册」
- 真机 e2e 两次绿（SearXNG 返回 9 条）；变异：不注入端点 / 关闭态仍注册 / 默认值改错 / 不并入端点主机 / 重定向用共享白名单 ⇒ 5 条全红

plugins/tools/io/doc.go

@@ -20,7 +20,13 @@
 //   - http: GET/POST only, timeouts, response truncation and SSRF protection
 //     (loopback/private/link-local addresses and localhost are refused unless
 //     the host is on an explicit allowlist).
-//   - web-search: requires a configured search endpoint, otherwise a clear error.
+//   - web-search: queries the configured SearXNG-compatible endpoint. When no
+//     usable endpoint is configured the composition root passes WithoutSearch, so
+//     the tool is not registered at all — the model never sees a tool that could
+//     only error. The endpoint itself is operator configuration (the model only
+//     supplies the query), hence its host is trusted by this tool alone: a
+//     loopback fallback endpoint stays reachable without widening the shared
+//     http/web-fetch allowlist.
 //   - web-fetch: size- and content-type-limited readable text extraction.
 //   - knowledge: RAG lookup over the runtime memory retriever; a clear message
 //     when no knowledge base is configured.