fix: checkout missing in workflow-variables job causes promote-branch to always attempt branch creation (#205)

The `workflow-variables` job in `promote-branch.yml` had no
`actions/checkout` step, so `origin` was never configured as a git
remote. `git ls-remote --exit-code --heads origin <target-branch>`
always failed (exit non-zero), and with `set +e` the error was swallowed
— resulting in `target-branch-exists` always being `false`. The
`promote-branch` job then unconditionally tried to create the target
branch from `origin/main`, which was rejected as a non-fast-forward push
when the branch already existed ahead of `main`.

## Changes

- **`.github/workflows/promote-branch.yml`**: Add `actions/checkout@v6`
as the first step in `workflow-variables` so `origin` is configured
before `git ls-remote` runs, enabling the `target-branch-exists` guard
to work correctly.

Author: Copilot

.github/workflows/promote-branch.yml

@@ -60,6 +60,8 @@ jobs:
         target-branch-exists: ${{ steps.check-target-branch-exists.outputs.target-branch-exists }}
         pull-request-exists: ${{ steps.check-pull-request-exists.outputs.pull-request-exists }}
     steps:
+      - name: 'Checkout ${{ github.head_ref || github.ref }}'
+        uses: actions/checkout@v6
       - name: 'Set target branch'
         id: set-target-branch
         run: |