fix: preserve workspace during branch creation in promote-branch workflow (#162)

`git switch <base-branch>` in the "Create target branch" step replaced
the workspace with the base branch contents. Since `main` doesn't carry
the composite actions, the immediately following `Lock target branch`
step couldn't resolve `action.yml`.

## Change

- **`promote-branch.yml`** — replaced the `git switch` + `git checkout
-b` + `git push` sequence with a single refspec push:
  ```sh
  git fetch origin
  git push origin origin/<base-branch>:refs/heads/<target-branch>
  ```
Creates the remote branch from the base ref without touching the local
workspace, so the source branch's composite actions remain accessible
for subsequent steps.

## Side effect to handle

`release/1.0` was already created by the failed run (off `main`,
unprotected). After this fix lands on `preview/1.0`, delete
`release/1.0` and re-run `promote-branch` to get a correctly locked
branch.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

.github/workflows/promote-branch.yml

@@ -205,13 +205,12 @@ jobs:
           dotnet-version: ${{ env.dotnet-sdk-version }}
       - name: 'Create target branch'
         if: ${{ needs.workflow-variables.outputs.target-branch-exists == 'false' }}
+        env:
+          base-branch: ${{ needs.workflow-variables.outputs.base-branch }}
+          target-branch: ${{ needs.workflow-variables.outputs.target-branch }}
         run: |
-          git config user.name "$(git log -n 1 --pretty=format:%an)"
-          git config user.email "$(git log -n 1 --pretty=format:%ae)"
           git fetch origin
-          git switch ${{ needs.workflow-variables.outputs.base-branch }}
-          git checkout -b ${{ needs.workflow-variables.outputs.target-branch }} origin/${{ needs.workflow-variables.outputs.target-branch }} || git checkout -b ${{ needs.workflow-variables.outputs.target-branch }}
-          git push --set-upstream origin ${{ needs.workflow-variables.outputs.target-branch }}
+          git push origin origin/${{ 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'