Add signed checksums, SBOMs, and provenance to release artifacts

[Vonng]

Summary

The release currently publishes SHA-256 checksums but does not provide:

• signed checksums or artifact signatures;
• SBOMs;
• build provenance/attestations;
• signed release tags.

Goal

Give downstream users verifiable evidence that binaries, packages, and images were built from the intended SILO Console source and dependency graph.

Acceptance criteria

• Generate an SPDX or CycloneDX SBOM for binaries/packages and OCI images.
• Publish build provenance for the tagged commit and workflow identity.
• Sign checksum manifests and container images using an agreed keyless or managed-key workflow.
• Verify signatures and attestations in CI before publishing a release.
• Document offline verification commands.
• Preserve a key-rotation/revocation procedure if managed keys are used.
• Decide whether annotated git tags must also be signed and enforce the decision.

[Vonng]

Resolved in SILO Console v2.4.1, published from 1360e26d976d82eda395b0b2e449df8c9d49f39c.

• The release includes SPDX SBOMs for the source/frontend dependency graph, all six binaries and all nine Linux packages. The checksum manifest binds the source, binaries, packages and their SBOMs to their digests.
• Checksums are signed with keyless Cosign using the GitHub Actions OIDC identity for release.yaml@refs/tags/v2.4.1. GitHub build provenance records the tagged source and workflow identity.
• The release workflow verified signatures, SBOM attestations and provenance before completing the draft. The exact release commit had already passed the complete CI matrix and vulnerability gates.
• After downloading the actual release assets, we independently verified the checksum signature and provenance, matched all 38 manifest entries to GitHub's asset digests, and verified the downloaded Darwin/arm64 executable's checksum, version, source commit and dependency metadata.
• The release contract documents online/offline verification and incident handling. Git tag signatures are explicitly optional; the mandatory signing identity is the release workflow at the exact tag and validated commit. There is no managed signing key to rotate.

This completes this issue's artifact-integrity requirements. Docker Hub image signing, public delivery and latest promotion are tracked separately in #34.