Adds guidance to prefer vertical slices (user-visible value) over horizontal slices (technical layers) when planning and writing issues. roadmap-planning skill: - New "Vertical vs Horizontal Slices" section - Demo test: "Can a user demo/test this independently?" - Good vs bad examples table - When horizontal slices are acceptable issue-writing skill: - New "Vertical Slices" section - Demo test guidance - Good vs bad issue titles table - User-focused issue framing examples Closes #31 Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>