dsh-exam-countdown: Add countdown for DSH exams in China

`dsh-exam-countdown` is an MIT-licensed DeepSeek Harness (DSH) plugin designed to enable Agents to directly query major Chinese exam dates and calculate the remaining days. It includes built-in data for 64 exams covering 9 major categories, such as college entrance exams, civil service exams, English proficiency tests, and teacher certifications. Adopting a pure ESM and zero-build architecture, it does not rely on a frontend environment. Its core functions include automatically adjusting expired dates, precisely calculating dates with specific rules (such as CET-4/6), and providing two tool interfaces: `exam_countdown` supports filtering and sorting results by ID, keyword, or category, while `exam_categories` is used to obtain the category list and counts. Installation requires adding the plugin to a specified profile and verifying the dependency configuration. This plugin is suitable for various exam schedule Q&A scenarios.