dsh-clock: Calendar and Wake-up Plugin for DeepSeek Harness

The `dsh-clock` plugin is designed to address the limitation of DeepSeek Harness’s scheduling feature, which is confined to the current session, by enabling cross-session scheduled wake-ups. The plugin supports setting one-time reminders through the Agent tool or the calendar panel and can trigger them even if the conversation has been closed. It employs a three-layer timing mechanism—in-process timers, the Windows Task Scheduler, and catch-up replay—to ensure wake-up reliability, and includes time drift injection and late warning features. Additionally, the plugin provides a visual calendar panel that supports branch reminder copying and a drift tolerance of up to 60 seconds.