WA-NEW-012: Update GitHub Actions CI Ruby to 2.7.8

[kitcommerce]

Summary

• Update GitHub Actions CI to run on Ruby 2.7.8 (workarea-core requires Ruby >= 2.7).
• Add pull_request trigger for PRs targeting next.
• Centralize the CI Ruby version in env.RUBY_VERSION.

Fixes #636.

CI Evidence

• CI run: (pending)

Client Impact

• Internal CI configuration change only; no runtime impact to client apps.
• Contributors will see CI run against Ruby 2.7.8 going forward.

[kitcommerce]

Dispatcher Build Gate Summary (local)

• rubocop (diff-only): N/A (no Ruby files changed)
• brakeman: skipped (per repo build gate config; disabled)
• tests: N/A (no engine code changed)

Labeled gate:build-passed. Ready for review.