dsh-first-maintenance: First Maintenance Reminder

`dsh-first-maintenance` is a pure Node.js plugin designed specifically for DeepSeek Harness (DSH) to implement vehicle first-maintenance reminders. It supports generating clear maintenance recommendations through parameter-based calculations and does not rely on external network access. Developers can install and use it with the `dsh plugin add` command, making it suitable for scenarios that require integrating maintenance logic into conversational systems.