Fix release workflow PR creation and merge strategy instruction

[Copilot]

The release workflow failed with 403 "Resource not accessible by integration" when creating PRs via curl. Additionally, the PR body incorrectly instructed squash-merge instead of rebase-merge.

Changes

Workflow permissions

• Added pull-requests: write to workflow-level permissions

PR creation

• Replaced curl-based GitHub API call with gh pr create
• Properly sets GH_TOKEN environment variable for authentication

Merge strategy instruction

• Changed PR body from "should be squash-merged" to "should be rebased and merged"

- name: Create Pull Request for Release
  run: |
    gh pr create \
      --title "Release version ${{ env.NEXT_VERSION }}" \
      --body "...\n\nThis PR should be rebased and merged into main." \
      --base main \
      --head "${{ env.BRANCH_NAME }}"
  env:
    GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.