dsh-exam-timing: Exam Time Allocation Plugin

The `dsh-exam-timing` plugin for DSH (DeepSeek Harness) is designed to provide a precise time allocation tool for exam simulation or task planning. Maintained by uckkk and implemented in pure Node.js, it has no network dependencies. Users can install it with the `dsh plugin add` command and directly invoke the `exam_timing` tool in a session to perform calculations. The plugin source code is hosted on GitHub, and it runs with the permissions of the current process.