ci: sign release checksums.txt and attach SBOM via GoReleaser #57

Summary

Consumers verifying by downloading the tarball + checksums.txt currently trust the unsigned checksums.txt via the attestation bundle alone. No SBOM is published, which is conspicuous for a library promoting supply-chain hardening.

Scope

• Add sboms: to .goreleaser.yml (syft-backed, CycloneDX JSON). Result uploaded as a release asset.
• Add the SBOM path to actions/attest-build-provenance's subject-path so it's signed alongside the tarballs.
• Document the verification flow in SECURITY.md.

Acceptance criteria

1. Release assets include a .cdx.json SBOM.
2. gh attestation verify <sbom>.cdx.json --owner axonops succeeds.
3. SECURITY.md describes the gh attestation verify flow for tarball, checksums, SBOM.

Source: devops + security-reviewer agents.