#3 の対応

IO/LocalStore.cs

@@ -29,6 +29,9 @@ public class LocalStore<T> : IStore<T>
             using (FileStream fs = new FileStream(this.filePath, FileMode.OpenOrCreate))
             using (StreamWriter writer = new StreamWriter(fs, DefaultEncoding))
             {
+                fs.Seek(0, SeekOrigin.Begin);
+                fs.SetLength(0);
+
                 foreach (T item in this.items)
                 {
                     string json = JsonSerializer.Serialize<T>(item);