SDD Plan-Scoped Workspace Implementation Plan

Goal: Make SDD's durable-progress workspace plan-scoped (.superpowers/sdd/ /) with a self-identifying ledger and end-of-plan cleanup, so a follow-up plan can never collide with a previous plan's artifacts and resumed controllers stop paying a forensic disambiguation tax. Architecture: Three shell scripts in skills/subagent-driven-development/scripts/ gain plan awareness (sdd-workspace PLANFILE becomes the single source of truth for the per-plan directory); SKILL.md's Durable