model-usage-plugin: DSH Model Consumption Statistics and Cost Estimation

`model-usage-plugin` is an MIT-licensed plugin designed for the DeepSeek Harness (DSH) web environment, aimed at addressing cost reconciliation issues under multi-model invocations. Its core functions include tracking token consumption by model (differentiating cache hits, misses, and writes), counting invocations and failures, and estimating costs based on built-in or custom pricing. It supports default pricing configurations for popular models, allowing users to modify unit prices, currencies, and peak/off-peak pricing periods. The plugin automatically retrieves and caches exchange rates, uniformly displaying costs in a target currency (default is RMB), and also supports querying and prominently displaying the DeepSeek account balance. Users can view details through the "Model Consumption" tab in the settings panel and check "Show only invoked models" to simplify the view. Developed in pure JS with zero build, the plugin integrates seamlessly via the `dsh plugin` system.