Add REFERENCE.md freshness check to pr_tests.yml (validate:strings)

[hcaballero2]

Adds a REFERENCE.md freshness job to the baseline pr_tests.yml running the read-only bundle exec rake validate:strings, so CI fails with a fix-it message whenever REFERENCE.md drifts from the source (and no-ops on modules without one). The next sync will also converge the 12 repos currently carrying the hand-rolled strings:generate:reference + git diff variant onto this single form. I deliberately left out the optional rake validate consolidation from #46 — replacing the separate syntax/metadata_lint steps is a bigger change and can be a follow-up. Draft because it's blocked on #42: the job pins checkout@v5/Ruby 2.7 to match the current baseline and will need a rebase onto the refreshed template versions once #42 merges.

Fixes #46

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