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.Data/Models/SubtaskEntity.cs

@@ -0,0 +1,11 @@
+namespace ClaudeDo.Data.Models;
+
+public sealed class SubtaskEntity
+{
+    public required string Id { get; init; }
+    public required string TaskId { get; init; }
+    public required string Title { get; set; }
+    public bool Completed { get; set; }
+    public int OrderNum { get; set; }
+    public required DateTime CreatedAt { get; init; }
+}