fix: fetch target branch before gh pr create --fill in promote-branch workflow

Agent-Logs-Url: https://github.com/petesramek/polyline-algorithm-csharp/sessions/06b9a62d-ceb7-4814-abd4-5263cae5f0c5

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

.github/workflows/promote-branch.yml

@@ -220,4 +220,5 @@ jobs:
           current-branch: ${{ github.ref_name }}
           target-branch: ${{ needs.workflow-variables.outputs.target-branch }}
         run: |
+            git fetch origin ${{ env.target-branch }}
             gh pr create --title "Promote ${{ env.current-branch }} to ${{ env.target-branch }}" --fill --base ${{ env.target-branch }} --head ${{ env.current-branch }}