feat(P0-001): scaffold monorepo structure

pnpm workspaces, Turborepo, TypeScript strict, ESLint flat config,
Prettier, Vitest, husky + lint-staged. Placeholder packages for all
16 packages + 2 apps + 2 plugins.

Closes #1

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -0,0 +1,26 @@
+{
+  "name": "mosaic-stack",
+  "private": true,
+  "packageManager": "pnpm@10.6.2",
+  "scripts": {
+    "build": "turbo run build",
+    "dev": "turbo run dev",
+    "lint": "turbo run lint",
+    "typecheck": "turbo run typecheck",
+    "test": "turbo run test",
+    "format": "prettier --write \"**/*.{ts,tsx,js,jsx,json,md}\"",
+    "format:check": "prettier --check \"**/*.{ts,tsx,js,jsx,json,md}\"",
+    "prepare": "husky"
+  },
+  "devDependencies": {
+    "@typescript-eslint/eslint-plugin": "^8.0.0",
+    "@typescript-eslint/parser": "^8.0.0",
+    "eslint": "^9.0.0",
+    "husky": "^9.0.0",
+    "lint-staged": "^15.0.0",
+    "prettier": "^3.0.0",
+    "turbo": "^2.0.0",
+    "typescript": "^5.8.0",
+    "vitest": "^2.0.0"
+  }
+}