spec-implement

Specification-driven implementation — Execute implementation from specs to PR. Reads issue-to-pr-workflow.md as playbook, enforces coding-rules.md as quality gates, tracks progress via tasks.md checkboxes with resume capability. English triggers: "Implement from spec", "Start implementation", "Execute spec tasks" 日本語トリガー: 「仕様書から実装」「実装を開始」「specタスクを実行」