sync does not update PR base branch after lower PR is merged

[Lenderson2]

After the bottom PR in a 3-branch stack was merged, running gh stack sync correctly:

• Detected the merge
• Fast-forwarded trunk to the latest main
• Rebased the remaining two branches onto main ("adjusted for merged PR")
• Pushed the rebased branches

However, it did not update the next PR's base branch on GitHub. The second PR still showed the merged branch as its base
instead of main. I had to manually run gh pr edit --base main to fix it.

Expected behavior: After sync detects a merged PR, it should update the base branch of the next PR in the stack to point
to trunk (or the next non-merged ancestor).

Stack structure:
main (trunk)
└── feature/step-1 → PR #1 (MERGED)
└── feature/step-2 → PR #2 (base still pointed to merged branch)
└── feature/step-3 → PR #3