feat: 增强联网搜索与网页抓取的联动

1. TOOL_USAGE_GUIDE 新增第 6 条：联网搜索联动规则
   - 要求 AI 在 web_search 后必须选择相关 URL 调用 web_fetch 抓取详情
   - 明确告知搜索摘要往往不完整，必须抓取原文才能给出准确答案
2. web_search 默认结果数从 5 提升到 8，给 AI 更多线索定位目标页面

src/renderer/services/tool-registry.ts

@@ -346,7 +346,7 @@ export const TOOL_DEFINITIONS: ToolDefinition[] = [
         required: ['query'],
         properties: {
           query: { type: 'string', description: 'The search query. Be specific and concise for best results.' },
-          max_results: { type: 'integer', description: 'Maximum number of results to return. Default: 5, max: 10.' }
+          max_results: { type: 'integer', description: 'Maximum number of results to return. Default: 8, max: 10.' }
         }
       }
     }