Frame

Structure implementation as vertical slices — each phase crosses all affected layers end-to-end, producing a testable integrated path. Phase 1 is the tracer bullet: the thinnest possible slice wired through every layer. Models default to horizontal plans (all DB, then all API, then all UI); this skill enforces vertical structure that cannot be prompted away.

Horizontal (wrong — layer by layer):

• Phase 1: All DB — notification tables, preferences, delivery log
• Phase 2: All service — email sender, in-app dispatcher, preference resolver
• Phase 3: All API — notification endpoints, preference endpoints
• Phase 4: All UI — notification center, preference panel, toast