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

vitest.workspace.ts

@@ -0,0 +1,3 @@
+import { defineWorkspace } from 'vitest/config';
+
+export default defineWorkspace([]);