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/probe/tests/MoShiro.Probe.Tests/ProbeTests.cs

@@ -82,4 +82,20 @@ public sealed class ProbeTests
             Directory.Delete(directory, recursive: true);
         }
     }
+
+    [Fact]
+    public void ConfigStore_CreatesSubdirectories_ForFreshVolumes()
+    {
+        // 新建的命名卷里没有 update/：面板转来的更新清单要能写进去
+        string directory = NewDirectory();
+        try
+        {
+            new ConfigStore(directory).Write("update/offer.manifest", "{}"u8.ToArray());
+            Assert.True(File.Exists(Path.Combine(directory, "update", "offer.manifest")));
+        }
+        finally
+        {
+            Directory.Delete(directory, recursive: true);
+        }
+    }
 }