feat(release): add SBOM-to-archive attestation linkage

[jmeridth]

Proposed Changes

Add a new attest_sboms matrix job that runs actions/attest-sbom per (archive, SBOM) pair after release_goreleaser produces artifacts. Expose sbom_matrix and is_public as outputs from release_goreleaser, upload dist/ as a workflow artifact when SBOMs are detected, and gate publish_release on attest_sboms so the draft publishes only after all SBOM attestations complete.

Why

The workflow already generates build provenance attestations against every artifact in dist/, including *.spdx.json files as standalone subjects, but it does not link an archive to its corresponding SBOM. Consumers running gh attestation verify --predicate-type https://spdx.dev/Document against a downloaded archive cannot discover its SBOM via the attestation graph. Adding actions/attest-sbom linkage closes that gap and removes the need for per-consumer follow-on jobs that replicate the matrix dance locally (downstream consumers have been writing this exact pattern in their callers).

Notes for reviewers

• New job runs only when create-attestation: true, the repo is public, and dist/*.spdx.json files exist. Behavior is unchanged when SBOMs are not produced or attestation is off.
• Matrix is generated from dist/*.spdx.json filenames, stripping .spdx.json to derive the archive path. Relies on GoReleaser's default ${artifact}.spdx.json naming pattern (or any pattern that maps 1:1 by the same suffix strip).
• publish_release now waits for attest_sboms to succeed-or-skip — mirrors the existing gates on release_goreleaser and release_image. Matrix uses fail-fast: false so one failed pair does not cancel the others.
• dist/ upload uses 1-day retention; consumed by the matrix job immediately, no long-term need.
• release_goreleaser continues to do attest-build-provenance over dist/*.spdx.json; the new SBOM linkage is additive, not a replacement.
• No new inputs or secrets — feature rides on the existing create-attestation flag. Considered adding a separate attest-sboms: boolean knob but it felt redundant given the existing flag's name.

Testing

• actionlint .github/workflows/release.yaml clean.
• YAML parses cleanly.
• test-release.yaml does not pass goreleaser-config-path, so release_goreleaser is skipped in tests; attest_sboms cascades to skipped via its if: guard — no test regression. End-to-end verification requires triggering a release in a downstream caller that exercises goreleaser + SBOMs (planned: ossf/pvtr-github-repo-scanner will consume this once released).

Readiness Checklist

Author/Contributor

• If documentation is needed for this change, has that been included in this pull request — docs/release.md updated with the new job and SBOM linkage behavior.

[jmeridth]

@zkoppert this has me thinking on how to split out specific release parts for specific languages. This release yaml is smelling of a bit too much Go IMO. 😄

For now, need this to be reviewed and merged for now. 🙏