feat: define thoughtbot rules for AI-enabled IDEs

rails/ai-rules/rails-rules.md

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+# IDE AI thoughtbot rules
+
+You are an expert in Ruby on Rails, PostgreSQL, and Hotwire (Turbo and Stimulus).
+
+## Key Conventions
+
+- Follow RESTful routing conventions: Seven restful actions: index, show, new, create, edit, update, delete ([thoughtbot post REST-ish routing guide](https://thoughtbot.com/blog/in-relentless-pursuit-of-rest-ish-routing))
+- Prefer classes to modules when designing functionality that is shared by multiple models.
+
+## Data / Models
+
+- To find model structure look in `db/schema.rb`
+- When working with model attributes don’t guess, grep the schema at `db/schema.rb` to confirm and use only valid attributes
+
+## UI and Styling
+
+- Use Rails view helpers and partials to keep views DRY
+
+## Performance Optimization
+
+- Always check for N+1 queries when rendering collections
+- Prefer includes for eager loading
+
+## Testing
+
+- Always write tests to cover new code generated
+- Use RSpec for testing framework, unless the project already uses minitest
+- Use factories ([FactoryBot](https://thoughtbot.github.io/factory_bot/))
+- In tests, avoid lets and before (avoid mystery guests), do test setup within each test
+- Verify new code by running test files using `bundle exec rspec spec/path/to/file_spec.rb`
+- You can run a specific test by appending the line number (it can be any line number starting from the "it" block of the test) eg. `bundle exec rspec spec/path/to/file_spec.rb:72`