Structured incremental development workflow that enforces explicit scoping before implementation. Use when: (1) User says 'build', 'implement', 'develop', 'create feature', or 'add functionality', (2) Task involves multiple files or could have ripple effects, (3) User wants to avoid scope creep or premature implementation, (4) Breaking down complex tasks into verifiable steps. This skill prevents AI agents from jumping into code without a validated plan.