Add SLSA generic generator workflow

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PR Summary

Introduce a GitHub Actions workflow that builds artifacts and generates SLSA level 3 provenance on releases.

• CI/CD — GitHub Actions:
  • Add /.github/workflows/generator-generic-ossf-slsa3-publish.yml to generate SLSA L3 provenance on release.
  • build job: checks out repo, creates artifacts, outputs base64-encoded SHA256 subjects.
  • provenance job: runs slsa-framework/slsa-github-generator reusable workflow (generator_generic_slsa3.yml@v1.4.0) with required permissions and uploads assets to the release.

^{Written by Cursor Bugbot for commit bf7499a. Configure here.}

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Bug: Output variable name mismatch breaks provenance generation

The step output variable name hashes doesn't match what the job output expects. Line 55 sets hashes=$(sha256sum...) but line 23 tries to read steps.hash.outputs.digests. This mismatch causes needs.build.outputs.digests to be empty, so the provenance job receives no digest data. Either the step should use digests= or the job output should reference steps.hash.outputs.hashes.

Additional Locations (1)

• .github/workflows/generator-generic-ossf-slsa3-publish.yml#L22-L23