Arn Spark Use Cases

- This skill should be used when the user says "use cases", "arn use cases", "write use cases", "define use cases", "Cockburn use cases", "actor goals", "behavioral requirements", "system behavior", "what does the app do", "describe the behavior", "use case document", "document the behavior", "define system behavior", or wants to create structured use case documents that describe the application's behavior from actor perspectives, producing a use-cases/ directory with individual Cockburn fully-dressed use case files and a README index.