fix: 添加 Explore agent 系统提示词模板并优化日志级别

- 添加 exploreAgentSystemPrompt 模板用于匹配 Claude Code Explore 子代理
- 将详细的 prompt 内容从 error 日志移至 warn 级别，减少日志噪音

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/utils/contents.js

@@ -84,6 +84,11 @@ const PROMPT_DEFINITIONS = {
     title: 'Claude Code Compact System Prompt',
     text: 'You are a helpful AI assistant tasked with summarizing conversations.'
   },
+  exploreAgentSystemPrompt: {
+    category: 'system',
+    title: 'Claude Code Explore Agent System Prompt',
+    text: "You are a file search specialist for Claude Code, Anthropic's official CLI for Claude."
+  },
   outputStyleInsightsPrompt: {
     category: 'output_style',
     title: 'Output Style Insights Addendum',