dsh-cache-billing: DSH Cache Billing Plugin

`dsh-cache-billing` is a DSH plugin developed by better-er, designed to address the lack of visibility into cumulative costs in long-context scenarios. Integrated into the context ring pop-up, it provides three levels of billing details—for the current step, current turn, and entire session—calculated in real time based on peak/off-peak rates. The plugin supports third-party relays, applies a unified DeepSeek-V4.1-Flash pricing model, and tracks cache hit/miss statistics. Once installed, users can view costs directly in the DSH Web interface, helping developers determine whether they need to switch windows.