dsh-schedule-enable: Enable DeepSeek Harness's Built-in Scheduler

`dsh-schedule-enable` is a pure configuration-layer plugin designed to enable built-in scheduled task capabilities for DeepSeek Harness agent developers. By injecting `@deepseek-ai/dsh-schedule` into the configuration, the Agent gains access to tools for creating, listing, and deleting persistent reminders. This plugin supports delayed, target-time, and fixed-frequency (minimum 5-minute interval) reminders, but requires a new session to take effect, and delivery is limited to the current session.