Kanban becomes a multi-agent orchestration platform with auto-decomposition, swarm topology, and worktree-per-task

What this changes for operators

• Operators who ran Kanban as a task board must now decide whether to adopt orchestrator auto-decomposition and swarm topology, which turn a queue into a self-spawning multi-agent fleet with new operating state to supervise.
• Per-task model overrides and worktree-per-task change the cost and isolation profile of every queued task; an operator must re-plan budget and concurrency.
• Verification path: deploy v0.15.0, queue a decomposable task, and confirm the orchestrator spawns the expected sub-agents in isolated worktrees before trusting it with real work.