Restore Lock target branch step in promote-branch workflow

Agent-Logs-Url: https://github.com/petesramek/polyline-algorithm-csharp/sessions/54e03ed7-d624-48df-a9a1-d8f0a44801b7

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

Author: Copilot

.github/workflows/promote-branch.yml

@@ -205,6 +205,12 @@ jobs:
         run: |
           git fetch origin
           git push origin origin/${{ env.base-branch }}:refs/heads/${{ env.target-branch }}
+      - name: 'Lock target branch'
+        if: ${{ needs.workflow-variables.outputs.target-branch-exists == 'false' }}
+        uses: './.github/actions/github/branch-protection/lock'
+        with:
+          branch: ${{ needs.workflow-variables.outputs.target-branch }}
+          token: ${{ secrets.GH_ADMIN_TOKEN }}
       - name: 'Create PR: "Promote ${{ env.current-branch }} to ${{ env.target-branch }}"'
         if: ${{ needs.workflow-variables.outputs.pull-request-exists == 'false' }}
         env: