feat(data): add task numbers schema, allocator, and backfill migration

TaskEntity.Number is a global, monotonically increasing, never-reused
integer (displayed as #123), allocated from AppSettingsEntity.NextTaskNumber
via a single UPDATE...RETURNING statement rather than MAX(number)+1, which
would reissue a deleted task's number. Both insert paths (TaskRepository.
AddAsync and CreateChildAsync) route through the new TaskNumberAllocator,
with a bounded retry on a unique-index collision. One migration adds the
columns, backfills existing rows in creation order, and creates the unique
index afterwards. Data-layer only; MCP/UI wiring is later slices.

src/ClaudeDo.Data/Repositories/TaskRepository.cs

@@ -26,8 +26,7 @@ public sealed class TaskRepository
             .MaxAsync(ct);
         entity.SortOrder = (maxSort ?? -1) + 1;
 
-        _context.Tasks.Add(entity);
-        await _context.SaveChangesAsync(ct);
+        await TaskNumberAllocator.AddWithNumberAsync(_context, entity, ct);
     }
 
     public async Task UpdateAsync(TaskEntity entity, CancellationToken ct = default)
@@ -50,6 +49,11 @@ public sealed class TaskRepository
         return await _context.Tasks.AsNoTracking().FirstOrDefaultAsync(t => t.Id == taskId, ct);
     }
 
+    public async Task<TaskEntity?> GetByNumberAsync(int number, CancellationToken ct = default)
+    {
+        return await _context.Tasks.AsNoTracking().FirstOrDefaultAsync(t => t.Number == number, ct);
+    }
+
     public async Task<List<TaskEntity>> GetByListIdAsync(string listId, CancellationToken ct = default)
     {
         return await _context.Tasks
@@ -309,8 +313,7 @@ public sealed class TaskRepository
             Model = ModelRegistry.NormalizeAlias(model),
             MaxTurns = maxTurns,
         };
-        _context.Tasks.Add(child);
-        await _context.SaveChangesAsync(ct);
+        await TaskNumberAllocator.AddWithNumberAsync(_context, child, ct);
         return child;
     }