feat(data): add subtasks table, repository and prompt integration

Per-task checklist backend: subtasks table with CASCADE delete,
SubtaskEntity + SubtaskRepository (connection-per-op, async), DI
registration in App and Worker, TaskRunner composes a '## Sub-Tasks'
markdown block into the Claude prompt when subtasks exist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.App/Program.cs

@@ -49,6 +49,7 @@ sealed class Program
         // Repositories
         sc.AddSingleton<ListRepository>();
         sc.AddSingleton<TaskRepository>();
+        sc.AddSingleton<SubtaskRepository>();
         sc.AddSingleton<TagRepository>();
         sc.AddSingleton<WorktreeRepository>();
 
@@ -66,7 +67,8 @@ sealed class Program
             sp.GetRequiredService<ListRepository>(),
             sp.GetRequiredService<GitService>(),
             sp.GetRequiredService<WorkerClient>(),
-            sp.GetRequiredService<TagRepository>()));
+            sp.GetRequiredService<TagRepository>(),
+            sp.GetRequiredService<SubtaskRepository>()));
         sc.AddSingleton<TaskListViewModel>(sp =>
         {
             var taskRepo = sp.GetRequiredService<TaskRepository>();