fix: replace {tools_description} placeholder in system prompt

pipeline_service/agent_loop_v2.py

@@ -261,15 +261,18 @@ class AgentExecutor:
             if skill_block:
                 prompt += f"\n\n{skill_block}"
 
-        # 注入工具列表
-        prompt += "\n\n## 可用工具\n"
+        # 注入工具列表（先构建，再替换占位符）
+        tools_text = ""
         if self._tool_registry:
-            prompt += self._tool_registry.to_text_description()
+            tools_text = self._tool_registry.to_text_description()
         else:
+            lines = []
             for t in self.config.tools:
                 if t.enabled:
                     params = ", ".join(f"{k}: {v}" for k, v in (t.parameters or {}).items())
-                    prompt += f"- {t.name}({params}): {t.description}\n"
+                    lines.append(f"- {t.name}({params}): {t.description}")
+            tools_text = "\n".join(lines)
+        prompt = prompt.replace("{tools_description}", tools_text)
 
         # 注入项目上下文
         try: