plan-feature-scaffold

Internal step of plan-feature: from an already-designed SPEC (product half designed), fill the engineering half and generate the planning artifact set scaled to the feature's size (XS/S → SPEC-only with ≥ 2 phases in the SPEC, last = Hardening & PR; M/L → full set with a hardening phase), freeze one compact ACCEPTANCE.md for every size, and register the roadmap entry. Docs only — never code.