plan-phase-v1

Work breakdown for a superseded v1 plan — it writes phases.md, not execution.md. Use when the user invokes /plan-phase-v1, or is continuing an existing v1 plan produced by /plan-init-v1; a plan carrying the Format: v2 marker belongs to /plan-phase. Reads the plan document and breaks it into ordered phases, proposes the phase structure to the user for approval, then creates a phase document (with checklist) for each phase and writes a new phases.md execution tracker alongside the plan. The original plan document is never modified.