fix: reduce verify job packages permission to read-only (#73)

## 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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Paradoxbound <paradoxbound@users.noreply.github.com>
Co-authored-by: Paradoxbound <paradoxbound@users.noreply.github.com>

Author: paradoxbound

.github/workflows/docker-publish.yml

@@ -137,7 +137,7 @@ jobs:
     if: needs.check-version.outputs.is-new-version == 'true'
     permissions:
       contents: read
-      packages: write
+      packages: read
 
     outputs:
       amd64_digest: ${{ steps.check.outputs.amd64_digest }}