chore: Add agentic coding tooling

.crosslink/rules/ruby.md

@@ -0,0 +1,47 @@
+### Ruby Best Practices
+
+#### Code Style
+- Follow Ruby Style Guide (use RuboCop)
+- Use 2 spaces for indentation
+- Prefer symbols over strings for hash keys
+- Use `snake_case` for methods and variables
+
+```ruby
+# GOOD: Idiomatic Ruby
+class UserService
+  def initialize(repository)
+    @repository = repository
+  end
+
+  def find_user(id)
+    @repository.find(id)
+  rescue ActiveRecord::RecordNotFound
+    nil
+  end
+end
+
+# BAD: Non-idiomatic
+class UserService
+  def initialize(repository)
+    @repository = repository
+  end
+  def findUser(id)  # Wrong naming
+    begin
+      @repository.find(id)
+    rescue
+      return nil
+    end
+  end
+end
+```
+
+#### Error Handling
+- Use specific exception classes
+- Don't rescue `Exception` (too broad)
+- Use `ensure` for cleanup
+
+#### Security
+- Use parameterized queries (ActiveRecord does this by default)
+- Sanitize user input in views (Rails does this by default)
+- Never use `eval` or `send` with user input
+- Use `strong_parameters` in Rails controllers