dsh-goal-quiescence: Add evidence gatekeeping for DeepSeek Harness goal-mode sub-agents

`dsh-goal-quiescence` is an MIT-licensed community plugin for DeepSeek Harness, aiming to address the issue where parent agents in goal-mode might prematurely complete a goal before child agents have terminated or read the results. The plugin implements a completion evidence gate that refuses to execute `update_goal(action=complete)` until an active child agent run has finished and the parent agent has explicitly read the final output. Its core capabilities include: logging child agent lifecycle; preventing goal completion before the run settles; forcing the parent agent to call `goal_quiescence_ack` to return the final output to the context; and querying the status of still-blocking completion via `goal_quiescence_status`.