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.

Beginne beim Konfliktwiederherstellungspfad von gh stack modify --continue und vergleiche ihn mit der anfänglichen Konfliktmeldung von gh stack modify. Reproduziere die im Issue beschriebene Sequenz mit mehreren Konflikten, einschließlich des Stagings der ersten Lösung, und überprüfe anschließend, dass der nächste Konflikt den Branch, die in Konflikt stehenden Dateien, den Fortschritt sowie Anweisungen zum Auflösen, Stagen und Fortsetzen anzeigt statt nur des Exit-Status.

Vom Indexierungsmodell aus dem Issue-Text verfasst.