feat: add Ruby and Rails rules

rules/ruby/testing.md

@@ -0,0 +1,51 @@
+---
+paths:
+  - "**/*.rb"
+  - "**/*.rake"
+  - "**/Gemfile"
+  - "**/test/**/*.rb"
+  - "**/spec/**/*.rb"
+  - "**/config/routes.rb"
+---
+# Ruby Testing
+
+> This file extends [common/testing.md](../common/testing.md) with Ruby and Rails specific content.
+
+## Framework
+
+- Use **Minitest** when the Rails app follows the default Rails test stack.
+- Use **RSpec** when it is already established in the project or the team has explicit production conventions around it.
+- Do not mix Minitest and RSpec inside the same feature area without a migration reason.
+
+## Test Pyramid
+
+- Put fast domain behavior in model, service, query, policy, and job tests.
+- Use request/controller tests for HTTP contracts, auth behavior, redirects, status codes, and response shapes.
+- Use system tests with Capybara for browser-critical flows only; keep them focused and stable.
+- Cover background jobs with unit tests for behavior and integration tests for queue/enqueue contracts.
+
+## Fixtures And Factories
+
+- Use Rails fixtures when they are the project default and the data graph is small.
+- Use `factory_bot` when scenarios need explicit object construction or complex traits.
+- Keep test data close to the behavior being asserted; avoid global fixtures that hide setup cost.
+
+## Commands
+
+Prefer project-local commands:
+
+```bash
+bin/rails test
+bin/rails test test/models/user_test.rb
+bundle exec rspec
+bundle exec rspec spec/models/user_spec.rb
+```
+
+## Coverage
+
+- Use SimpleCov when coverage is enforced; keep thresholds in CI and avoid gaming branch coverage with low-value tests.
+- Add regression tests for bug fixes before changing production code.
+
+## Reference
+
+See skill: `tdd-workflow` for the repo-wide RED -> GREEN -> REFACTOR loop.