fix: prevent project background from being sent on every message

Critical bug fix: Project background was being sent with EVERY message,
causing AI to respond to background instead of follow-up questions.

Problem:
- First message: User asks A, AI answers A 鉁?- Second message: User asks B, but prompt includes background again
- AI responds to background content, ignores question B 鉂?
Solution:
- Only send full prompt template (with project background) on FIRST message
- For follow-up messages, send ONLY user input (+ knowledge base if exists)
- Maintain conversation history properly

Updated: conversationService.assembleContext()
- Check if historyMessages.length === 0 (first message)
- First message: use renderUserPrompt() with all variables
- Follow-up: send userInput directly (optionally with knowledge base)

This ensures multi-turn conversations work correctly.

backend/src/services/conversationService.ts

@@ -157,12 +157,27 @@ export class ConversationService {
     // 反转顺序（最早的在前）
     historyMessages.reverse();
 
-    // 渲染用户Prompt模板
-    const renderedUserPrompt = agentService.renderUserPrompt(agentId, {
-      projectBackground,
-      userInput,
-      knowledgeBaseContext,
-    });
+    // 判断是否是第一条消息
+    const isFirstMessage = historyMessages.length === 0;
+
+    // 渲染用户Prompt
+    let userPromptContent: string;
+    
+    if (isFirstMessage) {
+      // 第一条消息：使用完整模板（包含项目背景）
+      userPromptContent = agentService.renderUserPrompt(agentId, {
+        projectBackground,
+        userInput,
+        knowledgeBaseContext,
+      });
+    } else {
+      // 后续消息：只发送用户输入和知识库上下文（如果有）
+      if (knowledgeBaseContext) {
+        userPromptContent = `${userInput}\n\n## 参考文献（来自知识库）\n${knowledgeBaseContext}`;
+      } else {
+        userPromptContent = userInput;
+      }
+    }
 
     // 组装消息数组
     const messages: Message[] = [
@@ -183,7 +198,7 @@ export class ConversationService {
     // 添加当前用户输入
     messages.push({
       role: 'user',
-      content: renderedUserPrompt,
+      content: userPromptContent,
     });
 
     return messages;