chore: scaffold mosaic monorepo (Phase 0)

- pnpm workspaces + Turborepo
- tsconfig.base.json strict ESM
- @mosaic/types stub package
- Woodpecker CI pipeline with Valkey service
- docs/PRD.md, docs/TASKS.md, CONTEXT.md

packages/types/src/index.ts

@@ -0,0 +1,2 @@
+// @mosaic/types - shared type definitions
+export type Placeholder = Record<string, never>;