Grant release job write access to publish GitHub releases

[Copilot]

The release job failed with a 403 while generating release notes. It used a custom PAT without the required access, and the job had no explicit release-write permission.

• Permissions: Grant contents: write only to the release job.
• Token: Use the built-in GITHUB_TOKEN instead of MY_REPO_PAT.

release:
  permissions:
    contents: write