Set up development environment: RuboCop rake task, RSpec, and SimpleCov

[eclectic-coding]

Summary

• Adds rake rubocop task and makes it the default alongside rake spec — bundle exec rake now runs the full suite
• Adds rspec-rails with a minimal dummy Rails app (spec/dummy/) for request specs against the mounted engine
• Adds simplecov configured with the Rails profile, grouping by Controllers / Helpers / Views / Library
• Adds a test job to CI running bundle exec rspec
• Fixes pagy v43 API throughout: Pagy::Backend/Frontend → Pagy::Method, pagy(collection) → pagy(:offset, collection), pagy_nav → @pagy.series_nav
• Requires solid_queue explicitly in the engine to ensure the constant is available in test environments

Test plan

• bundle exec rake runs rubocop (0 offenses) then rspec (2 examples, 0 failures)
• coverage/index.html is generated after running specs
• CI lint and test jobs both pass

🤖 Generated with Claude Code