feat(data): add Waiting task status and CreatedBy column

- New TaskStatus.Waiting for sequential subtask chains.
- New TaskEntity.CreatedBy column with migration AddTaskCreatedBy.
- TaskRepository.GetByCreatorAsync for filtering by creator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -75,6 +75,15 @@ public sealed class TaskRepository
     public Task<List<TaskEntity>> GetByListAsync(string listId, CancellationToken ct = default)
         => GetByListIdAsync(listId, ct);
 
+    public async Task<List<TaskEntity>> GetByCreatorAsync(string createdBy, CancellationToken ct = default)
+    {
+        return await _context.Tasks
+            .AsNoTracking()
+            .Where(t => t.CreatedBy == createdBy)
+            .OrderByDescending(t => t.CreatedAt)
+            .ToListAsync(ct);
+    }
+
     #endregion
 
     #region Status transitions