Install a Chinese exam countdown timer for DeepSeek Harness with dsh-exam-countdown

`dsh-exam-countdown` is a DeepSeek Harness community plugin maintained by zimai233, licensed under MIT. It turns 64 Chinese exams including the National College Entrance Examination, postgraduate entrance examination, national civil service examination, CET-4 and CET-6, CPA, National Judicial Examination and others into two tools: `exam_countdown` and `exam_categories`. It calculates the date and remaining days of the next exam purely through date arithmetic, and automatically rolls over to the next year after the exam date expires. The directory installation command is `dsh plugin add github:zimai233/dsh-exam-countdown`. The dates are for reference only, please check the source code and license before installation.