feat(build): publish OCI SBOM and provenance attestations

[alangou]

Summary

Publish per-platform SPDX SBOM and minimal SLSA provenance as OCI artifacts for pushed gateway and supervisor images. This PR is stacked on #2734 so release image SBOMs can include the cargo-auditable Rust dependencies introduced there.

Related Issue

Part of #2686.

Stacked on #2734.

Changes

• Enable BuildKit SBOM and minimal provenance attestations for registry pushes
• Export attestations with OCI media types and explicit subjects
• Verify the merged multi-architecture SBOM, including Cargo metadata for auditable images
• Document how to inspect published image SBOMs

Testing

• mise run pre-commit passes
• SBOM verification exercised against synthetic single- and multi-platform attestations, including missing SPDX and Cargo metadata
• Registry-backed Docker E2E (test:e2e label on this PR)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated

[github-actions]

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