dsh-countdown: Countdown Plugin

In the DeepSeek Harness ecosystem, the `dsh-countdown` plugin provides developers with countdown functionality that has no network dependencies. Implemented in Node.js, it registers a tool interface for DSH sessions to invoke. It supports lightweight time-logic extensions, requires the `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools` dependencies, and runs with the permissions of the current process.