Add release integrity checksums

[kamalsrini]

Summary

• Add a release workflow that packages release tags and generates SHA256SUMS.
• Attach checksum artifacts to GitHub releases and upload workflow artifacts for manual runs.
• Document release verification with sha256sum -c SHA256SUMS.

Notes

• This satisfies the roadmap acceptance with checksum-based release integrity. Key-managed signing can be layered on later without removing checksums.

Verification

• Parsed .github/workflows/release-integrity.yml as YAML.
• ruby scripts/validate_framework_registry.rb --stale --max-age-days 365 --as-of 2026-06-16
• ruby scripts/validate_skill_schema.rb
• ruby scripts/validate_index.rb
• ruby scripts/test_skill_fixtures.rb
• git diff --check

Linear: UNI-23