dsh-usage-vendor-stats: DeepSeek Harness API usage statistics by vendor dimension

When running agents in DeepSeek Harness (hereinafter referred to as DSH), multiple vendors are often connected: the official API, huoshan, hebox, tokenrhythm, etc. While persistent session logs always carry token accounting and provider/model sources, there is no default view that directly answers questions such as "how much was used by each vendor during this period, what is the cache hit rate, and the approximate cost calculated at unit price." The existing `dsh-usage-stats` plugin aggregates usage by workspace but lacks a vendor dimension. The `dsh-usage-vendor-stats` introduced below fills this gap by performing usage statistics with the vendor as the primary dimension. dsh-usag...