Modularize web-search-agent with strategy modules

Split inline search strategies into separate module files (github-debug,
general-web, academic-papers, chinese-tech, stackoverflow). Agent now
loads modules dynamically via Read tool before searching. Update install
instructions to include module files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agents/web-search-modules/stackoverflow.md

@@ -0,0 +1,9 @@
+# Stack Overflow Module
+
+> 从 web-search-agent.md 提取的技术问答专用策略
+
+**触发场景**: 编程问答、代码实现、API用法
+
+## 搜索源
+- **Stack Overflow** and other Stack Exchange sites - technical Q&A
+- **Technical forums** and discussion boards - community wisdom