feat: session-level systemPrompt takes priority over agent default

Co-authored-by: CodeFree <codefree@chinatelcom.cn>

server/service/agent/chat-engine.ts

@@ -248,7 +248,7 @@ export async function executeChat(params: ChatEngineParams): Promise<ChatEngineR
 
   const languageModel: LanguageModel = toLanguageModel(resolvedModel);
 
-  const systemPrompt = agent.systemPrompt || undefined;
+  const systemPrompt = session.systemPrompt?.trim() || agent.systemPrompt || undefined;
 
   const sessionEnableTools = session.enableTools !== null ? session.enableTools === 1 : agent.enableTools === 1;
   const toolsEnabled = bodyEnableTools !== undefined ? bodyEnableTools : sessionEnableTools;