Add kotlin-sum-types skill for parse-don't-validate pattern

Created new skill documenting the sealed classes pattern for type-safe
validation using the "parse, don't validate" principle.

What it teaches:
- Sealed classes for valid/invalid state representation
- @JsonCreator integration with Jackson for parsing
- Composable validation (Email → Address → RegistrationForm)
- Error collection with field paths
- Type-safe guarantees - invalid states unrepresentable in domain logic
- Controller pattern using when expressions

Based on working examples:
- src/main/kotlin/user/registration/RegistrationDomain.kt
- src/test/kotlin/user/registration/ParsingTest.kt

This pattern pushes validation to system boundaries and lets domain code
operate only on validated types, leveraging Kotlin's type system for safety.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: anderssv

README.md

@@ -36,6 +36,7 @@ If you're looking for the workshop, you can [find it here](doc/workshop/README.m
 
 This repository includes reusable [Claude Code skills](skills/README.md) that teach Claude the patterns and practices used here:
 
+- **kotlin-sum-types** 🔀 - Parse, don't validate with sealed classes for type-safe validation
 - **kotlin-tdd** 🧪 - Test-Driven Development with fakes, object mothers, and Testing Through The Domain
 - **kotlin-context-di** 🔌 - Manual dependency injection using SystemContext and TestContext patterns
 

skills/README.md

@@ -12,6 +12,29 @@ npx skills add anderssv/the-example/skills
 
 ## Available Skills
 
+### 🔀 kotlin-sum-types
+
+Parse, don't validate - using sealed classes for type-safe validation and state representation.
+
+**Location:** `domain/kotlin-sum-types/`
+
+**What it teaches:**
+- Sealed classes for representing valid/invalid states
+- Parse don't validate principle
+- Type-safe validation at boundaries
+- Composable validation (Email → Address → RegistrationForm)
+- Jackson integration with @JsonCreator
+- Error collection with paths
+- Making invalid states unrepresentable
+- Controller pattern for handling valid/invalid cases
+
+**Use when:**
+- Parsing external input (JSON, CSV, user forms)
+- Need to collect multiple validation errors
+- Want type-safe guarantees about data validity
+- Building domain models with complex validation
+- Integrating with JSON parsing libraries
+
 ### 🧪 kotlin-tdd
 
 Kotlin Test-Driven Development with fakes, object mothers, and Testing Through The Domain.