build

Implement one phase of a plan — reads the plan, finds the next incomplete phase, implements it with feedback loops, marks checkboxes, offers a commit (one phase per invocation). Use after /plan when a plan exists at .specs/plans/<slug>.md, or when the user asks to build or implement the next phase. Don't use for ad-hoc changes with no plan (use /tdd) or to check progress (use /test).