fix: create the update directory for fresh named volumes [CI]

ConfigStore.Write only created the config root, so a new volume without update/ made update offers fail with IO_PathNotFound. Create the parent directory on write and ship an empty update/ in the node images.

backend/fleet/src/Fleet.Hosting/Hosting.cs

@@ -243,7 +243,8 @@ public sealed class ConfigStore
             throw new InvalidOperationException("config files must stay inside the config directory");
         }
 
-        Directory.CreateDirectory(_directory);
+        // 相对路径里的子目录（例如 update/）也要建：新建的命名卷里只有镜像自带的目录
+        Directory.CreateDirectory(Path.GetDirectoryName(path)!);
         string temp = path + ".tmp";
 #pragma warning disable RS0030 // 纯采集边界的唯一豁免点：ConfigStore 只写配置目录，先写临时文件再原子替换
         File.WriteAllBytes(temp, content);