feat(core): add RSpec + Minitest helper files to Ruby group

Test harness bootstrap files loaded by every test in the suite:

  spec_helper.rb   — RSpec baseline configuration
  test_helper.rb   — Minitest baseline configuration
  rails_helper.rb  — Rails-specific RSpec bootstrap (require rails
                     env + all initialisers)

These sit under `spec/` or `test/` in canonical layouts (already
caught by dir rules), but Rails engines and multi-app monorepos
sometimes reference them via `require_relative` from other paths.
Kept as separate opt-in globs so projects that share helpers with
production code paths (rare, but legal) can leave them commented.