dsh-cost: Evidence-First Token Cost Ledger for DeepSeek Harness

dsh-cost is a plugin for DeepSeek Harness (DSH), designed to address the ambiguity of cost accounting caused by the mixing of information from different sources, by establishing an evidence-first token cost ledger based on persisted `assistant/message.usage` events. This plugin separates routing splits, evidence completeness, budget status, and current context pressure snapshots for clear presentation, and provides explicit fail-open/fail-closed budget checking mechanisms. Its core features include offline CLI cost calculation, a built-in stdio MCP server (exposing `cost_report` and `cost_check` interfaces), and a user-maintained price book support, with prices calculated per million tokens and differentiated between input and output.