feat: add intelligent backend routing and model service

- Add modelService for centralized model management
  - Support dynamic model list from config file (data/supported_models.json)
  - Include 2025 latest models: GPT-4.1, o3, o4-mini, Gemini 2.5, etc.
  - File watcher for hot-reload configuration changes

- Improve model detection logic in api.js
  - Priority: modelService lookup → prefix matching fallback
  - Smart backend routing based on model provider

- Add intelligent routing endpoints
  - /v1/chat/completions: unified OpenAI-compatible endpoint
  - /v1/completions: legacy format support
  - Auto-route to Claude/OpenAI/Gemini based on requested model

- Add Xcode system prompt support in openaiToClaude
  - Detect and preserve Xcode-specific system messages

- Export handler functions for reuse
  - openaiClaudeRoutes: export handleChatCompletion
  - openaiRoutes: export handleResponses

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

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

src/services/openaiToClaude.js

@@ -31,10 +31,25 @@ class OpenAIToClaudeConverter {
       stream: openaiRequest.stream || false
     }
 
-    // Claude Code 必需的系统消息
+    // 定义 Claude Code 的默认系统提示词
     const claudeCodeSystemMessage = "You are Claude Code, Anthropic's official CLI for Claude."
 
-    claudeRequest.system = claudeCodeSystemMessage
+    // 如果 OpenAI 请求中包含系统消息,提取并检查
+    const systemMessage = this._extractSystemMessage(openaiRequest.messages)
+    if (systemMessage && systemMessage.includes('You are currently in Xcode')) {
+      // Xcode 系统提示词
+      claudeRequest.system = systemMessage
+      logger.info(
+        `🔍 Xcode request detected, using Xcode system prompt (${systemMessage.length} chars)`
+      )
+      logger.debug(`📋 System prompt preview: ${systemMessage.substring(0, 150)}...`)
+    } else {
+      // 使用 Claude Code 默认系统提示词
+      claudeRequest.system = claudeCodeSystemMessage
+      logger.debug(
+        `📋 Using Claude Code default system prompt${systemMessage ? ' (ignored custom prompt)' : ''}`
+      )
+    }
 
     // 处理停止序列
     if (openaiRequest.stop) {