dsh-usage-cost: Calculate DeepSeek API cost per session per round

`dsh-usage-cost` is a DeepSeek Harness plugin designed to calculate API costs in real-time on the Web, round by round, per session. By projecting `usageTimeline` on the server side, the plugin records the token usage (including misses, outputs, and cache read/writes) and timestamps for each assistant turn. It provides cost readings across four dimensions in the chat interface: Conversation, Workspace, Today, and Last 5 Hours. Its core advantage is the automatic adaptation to the peak and valley pricing logic implemented since August 17th, which switches rates based on timestamps; if historical sessions lack timeline data, it falls back to flat-rate calculation. The plugin is distributed as an official bundle, requiring no additional building when installed from the Git repository—users simply need to execute the install command, restart the Web app, and refresh the page. Users can customize settings by editing `lib/client.js`.