fix(retail-store): configure Turbo build outputs for Next.js (#343)

## Problem

The root `turbo.json` declares `dist/**` as the expected build output
for all `build` tasks. However, `retail-store` is a Next.js app whose
`next build` outputs to `.next/`, not `dist/`. This causes Turborepo to
emit a warning on every build:

```
WARNING  no output files found for task retail-store#build. Please check your \`outputs\` key in \`turbo.json\`
```

## Fix

Add a package-level `turbo.json` in `examples/retail-store/` that
extends the root config and overrides the `build` task outputs to
`.next/**` (excluding `.next/cache/**`, which Next.js manages
internally).

This is the [standard Turborepo pattern for Next.js
apps](https://turbo.build/repo/docs/guides/frameworks/nextjs).

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Added a Turbo build pipeline configuration for the retail-store
example to standardize build inputs and outputs.
* Ensures build artifacts target the production output directory while
excluding build cache content to reduce artifact noise.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: aqrln

examples/retail-store/turbo.json

@@ -0,0 +1,19 @@
+{
+  "$schema": "https://turbo.build/schema.json",
+  "extends": ["//"],
+  "tasks": {
+    "build": {
+      "inputs": [
+        "src/**",
+        "app/**",
+        "public/**",
+        "middleware.ts",
+        "next.config.js",
+        "postcss.config.mjs",
+        "package.json",
+        "tsconfig.json"
+      ],
+      "outputs": [".next/**", "!.next/cache/**"]
+    }
+  }
+}