dsh-llm-cost: Add per-turn, per-step LLM cost display for DeepSeek Harness

`dsh-llm-cost` is a DeepSeek Harness (DSH) plugin designed to convert DSH-recorded token usage into specific dollar costs, addressing the issue where simply viewing token counts fails to intuitively reflect expenses. The plugin primarily generates a `costUsage` projection on the host side and renders the cost of each Turn and the cumulative session cost in real-time on the Web client, supporting model-based fee aggregation. For models without matched prices, the plugin displays `unknown` instead of $0.00 to avoid misunderstanding. Core features include cost detail display (including `totalCostUsd`, buckets, and step details), peak-valley pricing support (off-peak discounts during specific hours on weekdays), and automated price maintenance. Its `llm_c