fix: reduce verify job packages permission to read-only

[paradoxbound]

Summary

The verify job in docker-publish.yml only inspects existing images using docker buildx imagetools inspect — it never pushes. Reduces packages: write to packages: read to satisfy least-privilege (OSPS-AC-04.02).

All other job permissions were reviewed and are already correctly scoped:

• build-and-push: needs packages: write + id-token: write + attestations: write for pushing images and SLSA attestation
• scan: needs security-events: write for SARIF upload, packages: read for pulling image
• merge: needs contents: write for git tag + GitHub Release, packages: write for manifest creation
• cleanup: needs packages: write for deleting GHCR tags
• pre-merge-cd-check: needs packages: write for push/cleanup of PR test images

Test plan

• Pipeline passes on next merge to main

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