Agent Skills: java-concurrency

Implement modern concurrency with Virtual Threads and Structured Concurrency in Java. Use when implementing Java Virtual Threads (Java 21), Structured Concurrency with StructuredTaskScope, CompletableFuture pipelines, or debugging race conditions. (triggers: **/*.java, Thread, Executor, synchronized, lock, CompletableFuture, StructuredTaskScope, VirtualThread, AtomicInteger, async, race condition)