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

.woodpecker.yml

@@ -0,0 +1,30 @@
+steps:
+  - name: install
+    image: node:22-alpine
+    commands:
+      - corepack enable
+      - pnpm install --frozen-lockfile
+
+  - name: lint
+    image: node:22-alpine
+    commands:
+      - pnpm turbo lint
+
+  - name: typecheck
+    image: node:22-alpine
+    commands:
+      - pnpm turbo typecheck
+
+  - name: build
+    image: node:22-alpine
+    commands:
+      - pnpm turbo build
+
+  - name: test
+    image: node:22-alpine
+    commands:
+      - pnpm turbo test
+    services:
+      - name: valkey
+        image: valkey/valkey:8-alpine
+        ports: ["6379:6379"]