ci: declare contents:read on draft-release workflow

[arpitjain099]

Pins .github/workflows/draft-release.yml to permissions: contents: read at the workflow level. The job uses secrets.MATZBOT_AUTO_UPDATE_TOKEN for the commit step and for peter-evans/create-pull-request, so every operation that writes to the repository or opens a PR goes through that PAT and not GITHUB_TOKEN. The default token here is only used by the implicit actions/checkout calls (read on contents, read on the cross-org ruby/ruby clone).

Even though the writes are routed through a PAT, an explicit cap matters because of CVE-2025-30066 (the March 2025 tj-actions/changed-files compromise). A tampered third-party action exfiltrates GITHUB_TOKEN from workflow logs and the leaked token carries whatever scope was issued at the workflow level. Without an in-file declaration, that scope is whatever the org default is set to today, which can be broader than what this workflow actually needs. Pinning here makes the minimum enforceable, gives drift protection if the default ever widens, and registers with OpenSSF Scorecard's Token-Permissions check.

YAML validated locally with yaml.safe_load.