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

turbo.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://turbo.build/schema.json",
+  "tasks": {
+    "build": { "dependsOn": ["^build"], "outputs": ["dist/**"] },
+    "test": { "dependsOn": ["^build"] },
+    "lint": {},
+    "typecheck": { "dependsOn": ["^build"] }
+  }
+}