dsh-price-monitor: DeepSeek Harness Session Cost Monitoring

`dsh-price-monitor` is a session-level cost monitoring plugin for DeepSeek Harness (DSH). It does not provide a standalone interface but is rendered as a tab within `dsh-better-sidebar` to accurately track token counts and total costs per turn and per attempt. The plugin supports calculating costs based on official or manually configured pricing plans and allows refreshing official pricing. Before use, you must install and mount `dsh-better-sidebar` (>= 0.18.0) and select the corresponding billing plan.