docs: 添加 ToolSearch 设计指南 + 禁用 turn-zero 工具推荐弹窗

- 新增 docs/design/tool-search-design-guide.md，涵盖架构、搜索算法、执行管道、演进历史
- 禁用 getTurnZeroSearchExtraToolsPrefetch，消除用户输入时的频繁弹窗
- inter-turn 发现机制保持不变

Co-Authored-By: glm-5-turbo <zai-org@claude-code-best.win>

packages/builtin-tools/src/tools/WebFetchTool/WebFetchTool.ts

@@ -179,10 +179,10 @@ export const WebFetchTool = buildTool({
     }
   },
   async prompt(_options) {
-    // Always include the auth warning regardless of whether ToolSearch is
+    // Always include the auth warning regardless of whether SearchExtraTools is
     // currently in the tools list. Conditionally toggling this prefix based
-    // on ToolSearch availability caused the tool description to flicker
-    // between SDK query() calls (when ToolSearch enablement varies due to
+    // on SearchExtraTools availability caused the tool description to flicker
+    // between SDK query() calls (when SearchExtraTools enablement varies due to
     // MCP tool count thresholds), invalidating the Anthropic API prompt
     // cache on each toggle — two consecutive cache misses per flicker event.
     return `IMPORTANT: WebFetch WILL FAIL for authenticated or private URLs. Before using this tool, check if the URL points to an authenticated service (e.g. Google Docs, Confluence, Jira, GitHub). If so, look for a specialized MCP tool that provides authenticated access.