feat(ci): binary attestation with SLSA Build Provenance v1

[lockwobr]

Summary

• Attest CLI binaries with cosign keyless OIDC signing via GoReleaser build hook
• Distribute releases as tar.gz archives containing binary + attestation (.sigstore.json)
• Add on-demand Build Attested Binaries workflow for testing without release tags
• Update install script: tar.gz extraction, checksum verification, optional cosign attestation verification
• Extract SLSA predicate generation into reusable composite action

Motivation / Context

Fixes:
Related:

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• Build/CI/tooling

Component(s) Affected

• CLI (cmd/aicr, pkg/cli)
• API server (cmd/aicrd, pkg/api, pkg/server)
• Recipe engine / data (pkg/recipe)
• Bundlers (pkg/bundler, pkg/component/*)
• Collectors / snapshotter (pkg/collector, pkg/snapshotter)
• Validator (pkg/validator)
• Core libraries (pkg/errors, pkg/k8s)
• Docs/examples (docs/, examples/)
• Other: CI/Installer

Implementation Notes

Testing

# Commands run (prefer `make qualify` for non-trivial changes)
make qualify

• GoReleaser dry-run locally (hook skips, tar.gz created)
• build-attested.yaml workflow produces attested archives (verified from CI artifacts)
• Archive contents: {aicr, aicr-attestation.sigstore.json}
• Attestation is real Sigstore bundle with Rekor inclusion proof
• Tagged release produces attested tar.gz on GitHub Releases

Risk Assessment

• Low — Isolated change, well-tested, easy to revert
• Medium — Touches multiple components or has broader impact
• High — Breaking change, affects critical paths, or complex rollout

Rollout notes:

Checklist

• Tests pass locally (make test with -race)
• Linter passes (make lint)
• I did not skip/disable tests to make CI green
• I added/updated tests for new functionality
• I updated docs if user-facing behavior changed
• Changes follow existing patterns in the codebase
• Commits are cryptographically signed (git commit -S) — GPG signing info

[mchmarny]

/lgtm