feat: add Ruby and Rails rules

rules/ruby/coding-style.md

@@ -0,0 +1,46 @@
+---
+paths:
+  - "**/*.rb"
+  - "**/*.rake"
+  - "**/Gemfile"
+  - "**/*.gemspec"
+  - "**/config.ru"
+---
+# Ruby Coding Style
+
+> This file extends [common/coding-style.md](../common/coding-style.md) with Ruby and Rails specific content.
+
+## Standards
+
+- Target **Ruby 3.3+** for new Rails work unless the project already pins an older supported runtime.
+- Enable **YJIT** in production only after measuring boot time, memory, and request/job throughput.
+- Add `# frozen_string_literal: true` to new Ruby files when the project uses that convention.
+- Prefer clear Ruby over clever metaprogramming; isolate DSL-heavy code behind narrow, tested boundaries.
+
+## Formatting And Linting
+
+- Use the project's checked-in RuboCop config. For Rails 8+ apps, start from `rubocop-rails-omakase` and customize only where the codebase has a real convention.
+- Keep formatter/linter commands behind binstubs or scripts so CI and local runs match:
+
+```bash
+bundle exec rubocop
+bundle exec rubocop -A
+```
+
+- Do not silence cops inline unless the exception is narrow, documented, and harder to express cleanly in code.
+
+## Rails Style
+
+- Follow Rails naming and directory conventions before adding custom structure.
+- Keep controllers transport-focused: authentication, authorization, parameter handling, response shape.
+- Put reusable domain behavior in models, concerns, service objects, query objects, or form objects based on actual complexity, not as default ceremony.
+- Prefer `bin/rails`, `bin/rake`, and checked-in binstubs over globally installed commands.
+
+## Error Handling
+
+- Rescue specific exceptions. Avoid broad `rescue StandardError` blocks unless they re-raise or preserve enough context for operators.
+- Use `ActiveSupport::Notifications` or the app's logger for operational events; do not leave `puts`, `pp`, or `debugger` in committed application code.
+
+## Reference
+
+See skill: `backend-patterns` for broader service/repository layering guidance.