chore: Add agentic coding tooling

.crosslink/rules/scala.md

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+### Scala Best Practices
+
+#### Code Style
+- Follow Scala Style Guide
+- Prefer immutability (`val` over `var`)
+- Use case classes for data
+- Leverage pattern matching
+
+```scala
+// GOOD: Idiomatic Scala
+case class User(id: String, name: String)
+
+class UserService(repository: UserRepository) {
+  def findUser(id: String): Option[User] =
+    repository.find(id)
+
+  def processUser(id: String): Either[Error, Result] =
+    findUser(id) match {
+      case Some(user) => Right(process(user))
+      case None       => Left(UserNotFound(id))
+    }
+}
+```
+
+#### Error Handling
+- Use `Option` for missing values
+- Use `Either` or `Try` for operations that can fail
+- Avoid throwing exceptions in pure code
+
+```scala
+// GOOD: Using Either for errors
+def parseConfig(json: String): Either[ParseError, Config] =
+  decode[Config](json).left.map(e => ParseError(e.getMessage))
+
+// Pattern match on result
+parseConfig(input) match {
+  case Right(config) => useConfig(config)
+  case Left(error)   => logger.error(s"Parse failed: $error")
+}
+```
+
+#### Security
+- Use prepared statements for database queries
+- Validate input with refined types when possible
+- Never interpolate user input into queries