docs: record vertical slice reorder in scratchpad

Jason directed: build Pi TUI → Gateway → Discord communication
spine before backfilling horizontal layers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/scratchpads/mvp-20260312.md

@@ -47,4 +47,26 @@ User confirmed: start the planning gate.
 
 ## Corrections
 
-(none at this time)
+### 2026-03-13 — Vertical slice reorder (Jason directed)
+
+**Original plan:** Linear Phase 0 → 1 → 2 → ... execution.
+
+**Correction:** Vertical slice first. Scaffold monorepo, then build the Pi TUI → Gateway → Discord communication spine end-to-end before backfilling auth, brain, memory, CRUD, etc.
+
+**Why:** Validate the architecture's core message flow before investing in horizontal layers. If the communication channels don't work, nothing else matters.
+
+**Revised execution sequence:**
+
+| Step | Tasks (cross-phase) | What it proves |
+|------|---------------------|----------------|
+| 1 | P0-001: Scaffold monorepo | Build system works |
+| 2 | P0-005: Docker Compose (PG + Valkey) | Infrastructure runs |
+| 3 | P0-002: @mosaic/types (minimal — gateway, agent, chat types) | Shared contracts |
+| 4 | P1-001: Gateway scaffold (minimal NestJS + Fastify) | API surface boots |
+| 5 | P1-007: WebSocket server (chat streaming) | Real-time channel works |
+| 6 | P1-008: Basic agent dispatch (single provider) | LLM responds |
+| 7 | P2-001: @mosaic/agent — Pi SDK integration (minimal) | Pi sessions work |
+| 8 | P6-005: Pi TUI integration (mosaic tui → gateway) | TUI ↔ Gateway proven |
+| 9 | P5-001: Plugin host (channel plugin interface) | Plugin arch works |
+| 10 | P5-002: Discord plugin (bot + channel) | Discord ↔ Gateway proven |
+| — | Then backfill: auth, brain, db, queue, OTEL, CI, web dashboard, etc. |