ci: qualify private GHCR attestations

[backnotprop]

Summary

• authenticate the attestation job to private GHCR
• separate private image publication from npm/GitHub release publication
• add an explicit private-registry workflow mode using isolated ci-* image tags

Verification

• pnpm verify:iteration
• private workflow run: https://github.com/plannotator/artifact-server/actions/runs/33186452150
• private image push, digest verification, provenance attestation, amd64 SBOM attestation, and arm64 SBOM attestation passed
• npm publication and GitHub prerelease jobs were skipped
• GHCR package visibility remains private
• no v0.1.0 tag or GitHub release exists