Use GitHub API to get PR commits for cherry-pick

Instead of computing the revision range between the sync target branch
and the PR head (which includes unrelated commits from the base branch),
use the GitHub API to get only the commits that belong to the PR.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: holly-cummins

.github/workflows/syncbot.yml

@@ -26,6 +26,7 @@ jobs:
         id: cherry-pick
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           git config user.name syncbot
           git config user.email github-actions@github.com
@@ -36,7 +37,6 @@ jobs:
           
           # Get the source branch name and target branch
           SOURCE_BRANCH="${{ github.head_ref }}"
-          SOURCE_SHA="${{ github.event.pull_request.head.sha }}"
           TARGET_BRANCH="${{ steps.target-branch.outputs.branch }}"
           SYNC_BRANCH="sync-${SOURCE_BRANCH}-to-${TARGET_BRANCH}"
           
@@ -55,8 +55,8 @@ jobs:
           # Create sync branch from target branch (will reset if remote exists)
           git checkout -b $SYNC_BRANCH origin/$TARGET_BRANCH
           
-          # Get all commits from the PR
-          COMMITS=$(git log --reverse --format="%H" origin/$TARGET_BRANCH..$SOURCE_SHA)
+          # Get only the commits that are part of the PR
+          COMMITS=$(gh api repos/${{ github.repository }}/pulls/${{ github.event.pull_request.number }}/commits --jq '.[].sha')
           
           # Cherry-pick each commit
           echo "Cherry-picking commits..."