dsh-message-datetime: DeepSeek Harness Per-Turn Clock Context Plugin

`dsh-message-datetime` is a DeepSeek Harness plugin designed to address time perception and idle-duration calculation for large models in multi-turn conversations. At the start of each turn, it automatically injects the current time, the end time of the previous turn, and the idle duration, eliminating the need for the model to calculate them, thereby reducing token consumption and improving accuracy. It supports full scenario coverage and features zero permissions, interruption safety, and Web UI compatibility, along with timezone configuration options.