chore: Add agentic coding tooling

.crosslink/rules/kotlin.md

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+### Kotlin Best Practices
+
+#### Code Style
+- Follow Kotlin coding conventions
+- Use `val` over `var` when possible
+- Use data classes for simple data holders
+- Leverage null safety features
+
+```kotlin
+// GOOD: Idiomatic Kotlin
+data class User(val id: String, val name: String)
+
+class UserService(private val repository: UserRepository) {
+    fun findUser(id: String): User? =
+        repository.find(id)
+
+    fun getOrCreateUser(id: String, name: String): User =
+        findUser(id) ?: repository.create(User(id, name))
+}
+```
+
+#### Null Safety
+- Avoid `!!` (force non-null); use safe calls instead
+- Use `?.let {}` for conditional execution
+- Use Elvis operator `?:` for defaults
+
+```kotlin
+// GOOD: Safe null handling
+val userName = user?.name ?: "Unknown"
+user?.let { saveToDatabase(it) }
+
+// BAD: Force unwrapping
+val userName = user!!.name  // Crash if null
+```
+
+#### Coroutines
+- Use structured concurrency with `CoroutineScope`
+- Handle exceptions in coroutines properly
+- Use `withContext` for context switching
+
+#### Security
+- Use parameterized queries
+- Validate input at boundaries
+- Use sealed classes for exhaustive error handling