feat(release): sign release artifacts and add build provenance

[joshua-temple]

Problem

cascade publishes GitHub Releases via GoReleaser but attaches no signatures, no SBOM, and no build provenance, and the build is not reproducible (release ldflags embed a wall-clock timestamp). This blocks the OpenSSF Scorecard Signed-Releases check and the OpenSSF Best Practices signed_releases and build_reproducible criteria.

Fix

• Reproducible build: add -trimpath, set mod_timestamp to the commit timestamp, and switch the embedded date ldflag from build time to the commit date.
• cosign keyless signing (OIDC + Sigstore) of checksums.txt, emitting .sig and .pem as release assets.
• GPG detached signing of checksums.txt (.asc) using a passphraseless release key imported from the CASCADE_RELEASE_GPG_KEY repo secret. The public key is published at docs/cascade-release-public-key.asc.
• SBOM generation (syft, SPDX JSON) per archive.
• SLSA build provenance via actions/attest-build-provenance, verifiable with gh attestation verify.
• Least-privilege job permissions (id-token: write, attestations: write only on the release job).
• docs/release-verification.md: how to verify cosign, GPG, and provenance, and how to reproduce the build.

All new actions are SHA-pinned and verified to resolve.

Verification

• go build ./..., go test ./..., golangci-lint run ./... all pass.
• goreleaser check valid; actionlint clean on the workflow.
• Signing and SBOM only execute on a real release, so they will be exercised by the next rc.

Maintainer setup (done)

The signing key is provisioned: a passphraseless RSA-4096 key is generated, the CASCADE_RELEASE_GPG_KEY and CASCADE_RELEASE_GPG_FINGERPRINT repo secrets are set, and the public key is committed at docs/cascade-release-public-key.asc. The key has no passphrase by design (a passphrase would live as a repo secret in the same trust boundary), and is disposable: regenerate and republish the public key if needed.

Note on Scorecard score

cosign .sig/.pem and GPG .asc ship as release assets, satisfying Signed-Releases (signature credit). Build provenance is stored in GitHub's attestation API rather than as a .intoto.jsonl release asset; reaching the full provenance credit would require a tag-pinned generator that conflicts with SHA-pinning, so it is intentionally not pursued.