v1.5.0

Full supply-chain components for Go projects:

- go-audit: lint-stage govulncheck. allow_failure: true.
- go-sbom: tag-only CycloneDX SBOM per binary via syft.
- slsa-attest: tag-only SLSA v1.0 provenance per binary,
  keyless-signed via cosign + GitLab OIDC. Predicate field set
  matches public-sector-tools' attest template; verify with
  cosign verify-blob-attestation --type slsaprovenance1 or
  slsa-verifier.
- release-create: updated to link SBOMs and SLSA bundles
  alongside binaries on the GitLab Release page.

Combined with v1.4.0's go-build, go-release-binary, and the
existing release-create, any Go project in the dunn.dev estate
can now compose a full branch-and-tag CI from catalog includes:
audit + build on branches; package + sbom + attest + release on
tags.

First consumer: dunn.dev/bairn at v0.1.0.