Verify signatures, provenance, and SBOM attestations after publish

[ocularminds]

Context

Release jobs create provenance, SBOM attestations, and keyless cosign signatures, but post-publish validation currently confirms only that the image manifest exists.

Scope

Make supply-chain verification an explicit post-publish gate for all Docker Hub images.

Acceptance criteria

• Validation verifies the cosign signature against the expected GitHub Actions OIDC identity and issuer.
• Provenance is present and binds the published digest to this repository and workflow.
• An SBOM attestation is present for every published platform.
• Validation uses the immutable digest, not only a mutable tag.
• A failed verification makes the release workflow fail visibly.