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

package.json

@@ -0,0 +1,18 @@
+{
+  "name": "mosaic",
+  "private": true,
+  "scripts": {
+    "build": "turbo build",
+    "test": "turbo test",
+    "lint": "turbo lint",
+    "typecheck": "turbo typecheck",
+    "format": "prettier --write ."
+  },
+  "devDependencies": {
+    "turbo": "^2",
+    "@changesets/cli": "^2",
+    "typescript": "^5",
+    "prettier": "^3"
+  },
+  "packageManager": "pnpm@10.28.1"
+}