dsh-leap-year: Leap Year Check

The DSH (DeepSeek Harness) plugin `dsh-leap-year` is a lightweight tool for determining leap years, maintained by user uckkk. It is implemented in pure Node.js, has no network dependencies, and is suitable for offline use. The plugin provides basic year validation and must be installed in the DSH terminal using the specified command, with its dependencies primarily sourced from the DeepSeek ecosystem.