gh stack modify --continue hides subsequent rebase conflict details

## Description

When `gh stack modify --continue` advances an in-progress rebase and encounters another conflict, it prints only:

```text
✗ rebase continue failed — resolve remaining conflicts and try again: exit status 1
```

It does not show:

- Which branch is being rebased
- Which files conflict
- Rebase progress
- The usual resolve/stage/continue instructions

The initial `gh stack modify` conflict reports this information correctly. Subsequent conflicts reached through `--continue` do not.

## Reproduction

1. Create a stack where dropping or moving a lower branch causes conflicts in multiple downstream commits.
2. Run `gh stack modify`.
3. Apply the structural change.
4. Resolve the first conflict.
5. Stage the resolved file:

```bash
git add
```

6. Continue:

```bash
gh stack modify --continue
```

7. Let the rebase encounter another conflict.

## Actual result

Only the generic exit-status message appears:

```text
✗ rebase continue failed — resolve remaining conflicts and try again: exit status 1
```

The user must separately run:

```bash
git status
git diff --name-only --diff-filter=U
```

to discover that another conflict appeared and identify its files.

## Expected result

`gh stack modify --continue` should report the next conflict like the initial modify operation:

```text
Conflict rebasing

Conflicted files:

Resolve the conflicts, stage with `git add `, then run
`gh stack modify --continue`.
```

Progress for successfully completed branches or commits should also remain visible.

## Environment

- `gh` 2.97.0
- `gh-stack` 0.1.0
- macOS 26.6

## Additional context

This appears distinct from #146. No merged or pruned branch lookup is involved at this stage; the modify operation is already in conflict recovery and `--continue` reaches another conflicting commit.

從 gh stack modify --continue 的衝突復原路徑開始，並將其與 gh stack modify 初始的衝突報告進行比較。重現 issue 中描述的多重衝突流程，包括暫存第一個解決結果，然後確認下一個衝突會顯示分支、衝突檔案、進度，以及解決、暫存和繼續的指示，而不只是結束狀態。

由索引模型根據 Issue 內容生成。