Fix minimum token permissions for workflows that perform git push

[Copilot]

This PR adds the required contents: write permission to GitHub Actions workflows that perform git push operations.

Changes Made

Added permissions: contents: write to the following workflow jobs that perform git push operations:

• prepare-release-branch.yml:

  • create-pull-request-against-release-branch job (performs git push at lines 78, 105)
  • create-pull-request-against-main job (performs git push at line 196)

• backport.yml:

  • backport job (performs git push at line 41)

• prepare-patch-release.yml:

  • prepare-patch-release job (performs git push at line 79)

Why This Change Is Needed

GitHub Actions workflows need explicit contents: write permission to push changes to repositories. Without this permission, the git push operations in these workflows would fail with permission errors.

All additions include the trailing comment # required for pushing changes as specified in the issue requirements.

Verification

• ✅ All 5 git push operations across the repository are now covered
• ✅ YAML syntax validation passed
• ✅ Changes are minimal and surgical (only 8 lines added across 3 files)
• ✅ No existing functionality is modified

Fixes #34.

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