dsh-session-cost: Display current API costs in a DSH session

`dsh-session-cost` is a dedicated plugin designed specifically for DeepSeek Harness (DSH) to display model invocation costs in real-time, addressing the difficulty of verifying single-session costs when relying solely on billing records. The plugin displays session cumulative costs and per-turn costs in RMB without requiring API key configuration or setting modifications, taking effect immediately after installation and restart. Its core features include distinguishing between cache hits, misses, and output tokens, applying peak (2x) or off-peak pricing based on request time, and supporting the recalculation of historical session costs from full logs. The plugin includes up-to-date RMB pricing for DeepSeek-V4-Flash and Pro, ensuring data accuracy and preventing double billing. Users can conveniently install and uninstall the plugin via the `dsh` CLI or source code. It is suitable for use with DSH.