Refactor template serialization to use TemplateRegistry

Replaced the serialization of the `templates` object with a `TemplateRegistry` object initialized with `templates`. This change alters the data structure before serialization to JSON, potentially impacting how data is stored in the `templateRegistryPath` file.

Author: matt-goldman

tests/Blake.IntegrationTests/Commands/BlakeNewCommandTests.cs

@@ -44,7 +44,9 @@ public async Task BlakeNew_WithListOption_ShowsAvailableTemplates()
                 new (Guid.Empty, shortName2, longName2, "", "", "", DateTime.MinValue, "")
             };
 
-            var jsonContent = System.Text.Json.JsonSerializer.Serialize(templates, new System.Text.Json.JsonSerializerOptions { WriteIndented = true });
+            var registry = new TemplateRegistry(templates);
+
+            var jsonContent = System.Text.Json.JsonSerializer.Serialize(registry, new System.Text.Json.JsonSerializerOptions { WriteIndented = true });
             Directory.CreateDirectory(Path.GetDirectoryName(templateRegistryPath)!);
             File.WriteAllText(templateRegistryPath, jsonContent);
         }