dsh-session-budget-guard: DeepSeek Harness Session Tree Budget Circuit Breaker

`dsh-session-budget-guard` is a session-tree-level budget and spend circuit-breaker plugin for DeepSeek Harness, designed to prevent API quota surges caused by recursive subagent calls. It provides spend monitoring, circuit-breaker rejection, structural restrictions, and custom notifications. By configuring thresholds for cumulative spend, rate, and tree depth, developers can observe logs in `notify` mode and then enable `enforce` mode for hard stop-loss control. Note that the plugin runs in-process; budget records are lost after a restart, and branch statistics rely on stale sample data.