dsh-plugin-token-usage: Cross-session statistics of DSH token usage

`dsh-plugin-token-usage` is a DSH plugin (v0.2.0, MIT) maintained by lovedheart, designed to track token usage across sessions. It uses `~/.dsh/sessions` session logs as the single source of truth, bucketing and aggregating by (date, provider, model), covering all persistent sessions including subagents and historical data prior to installation. It achieves precise tracking through persistent watermarks and deduplication.