fix: fetch target branch before gh pr create --fill in promote-branch workflow (#206)

`gh pr create --fill` computes the PR body by running `git log
origin/<target>...HEAD`, but the shallow checkout (`fetch-depth: 1`)
only fetches the source branch. With `origin/<target>` absent from the
local repo, git fails with "ambiguous argument" and the job exits
non-zero.

## Changes

- **`.github/workflows/promote-branch.yml`** — add `git fetch origin ${{
env.target-branch }}` immediately before `gh pr create` in the "Create
PR" step, ensuring the remote tracking ref exists when `--fill` computes
the commit range.

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 }}