init

Initialize a sprint with kanban-style structure for multi-agent collaboration. Use when user wants to start a sprint, set up sprint structure, or manage work with multiple agents. Triggers include "new sprint", "init sprint", "setup sprint", "start sprint", "스프린트 시작", "스프린트 초기화", "새 스프린트". Creates BACKLOG.md, HANDOFF.md, INSTRUCTION.md, and sets up active/, refs/designs/, refs/plans/ folders.