Feature request: support merge-based stack synchronization without force-pushing

## Summary

Please add an option for stacked pull requests to synchronize branches using merge commits instead of rebasing and force-pushing branch history.

## Motivation

Our organization does not allow force-pushes because they rewrite branch history and can make audit trails, approvals, and incident investigation more difficult. The current "Rebase stack" workflow is therefore incompatible with our repository policies.

## Proposed behavior

Add a merge-based synchronization mode for stacks:

- When a parent branch changes, merge the parent into each child branch in stack order.
- Preserve existing commits and branch history.
- Do not force-push rewritten branch history.
- Keep the existing rebase behavior as the default or as a selectable repository/stack setting.
- Make the selected synchronization mode visible in the stack UI and CLI.
- Respect branch protection and merge-commit policies.

## Example

Given:

```text
main
└── feature-a
└── feature-b
```

When main advances, merge main into feature-a, then merge the updated feature-a into feature-b, instead of rebasing both branches and force-pushing them.

## Acceptance criteria

- A stack can be configured to use merge-based synchronization.
- Synchronizing the stack does not require force-pushing any branch.
- Existing approvals and checks are handled consistently with the merge-based history.
- Conflicts are reported per branch and can be resolved through the normal workflow.
- The behavior works with protected branches that block force-pushes.
- Existing rebase-based stacks continue to work unchanged.

## Related issues

- #268: Stacked PR cannot be merged into a merge-queue branch
- #442: gh stack sync pushed a stack branch's merge commit directly onto the trunk branch

Bắt đầu bằng cách truy vết workflow "Rebase stack" hiện có và lệnh đồng bộ stack trong CLI. Lập bản đồ cách xử lý việc cập nhật branch, Force-Pushes, xung đột, phê duyệt, checks và các branch được bảo vệ. Công việc được xem là hoàn tất khi một chế độ dựa trên Merge có thể chọn đồng bộ các branch theo thứ tự của stack mà không thực hiện Force-Pushes, trong khi các stack hiện có dựa trên Rebase vẫn tiếp tục hoạt động.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.