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.Worker/Runner/TaskRunner.cs

@@ -12,6 +12,7 @@ public sealed class TaskRunner
     private readonly TaskRunRepository _runRepo;
     private readonly ListRepository _listRepo;
     private readonly WorktreeRepository _wtRepo;
+    private readonly SubtaskRepository _subtaskRepo;
     private readonly HubBroadcaster _broadcaster;
     private readonly WorktreeManager _wtManager;
     private readonly ClaudeArgsBuilder _argsBuilder;
@@ -24,6 +25,7 @@ public sealed class TaskRunner
         TaskRunRepository runRepo,
         ListRepository listRepo,
         WorktreeRepository wtRepo,
+        SubtaskRepository subtaskRepo,
         HubBroadcaster broadcaster,
         WorktreeManager wtManager,
         ClaudeArgsBuilder argsBuilder,
@@ -35,6 +37,7 @@ public sealed class TaskRunner
         _runRepo = runRepo;
         _listRepo = listRepo;
         _wtRepo = wtRepo;
+        _subtaskRepo = subtaskRepo;
         _broadcaster = broadcaster;
         _wtManager = wtManager;
         _argsBuilder = argsBuilder;
@@ -91,9 +94,16 @@ public sealed class TaskRunner
             await _broadcaster.TaskStarted(slot, task.Id, now);
 
             // Build prompt.
-            var prompt = string.IsNullOrWhiteSpace(task.Description)
-                ? task.Title
-                : $"{task.Title}\n\n{task.Description.Trim()}";
+            var subtasks = await _subtaskRepo.GetByTaskIdAsync(task.Id, ct);
+            var sb = new System.Text.StringBuilder(task.Title);
+            if (!string.IsNullOrWhiteSpace(task.Description)) sb.Append("\n\n").Append(task.Description.Trim());
+            if (subtasks.Count > 0)
+            {
+                sb.Append("\n\n## Sub-Tasks\n");
+                foreach (var s in subtasks)
+                    sb.Append(s.Completed ? "- [x] " : "- [ ] ").Append(s.Title).Append('\n');
+            }
+            var prompt = sb.ToString();
 
             // Run 1.
             var result = await RunOnceAsync(task.Id, slot, runDir, resolvedConfig, 1, false, prompt, ct);