Add onboarding doc-test harness

Disposable rig that runs the global onboarding-tester agent against the
developer SDK-integration journey: boots a fresh keysat fixture, mints a
merchant-onboard scoped key, serves keysat-docs as the published corpus,
scaffolds a pristine Next.js/TS proof-of-work, and has the agent gate it
docs-only. Stage 1 (no payments) reached completed-clean over three runs;
see onboarding-harness/STAGE1-RESULT.md. Stage 2 (regtest buyer-pays) is
gated on the agent-payment-connect scope work.

onboarding-harness/sandbox-template/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "acme-reports",
+  "version": "0.1.0",
+  "private": true,
+  "description": "Pristine proof-of-work app for the Keysat onboarding harness. A tiny Next.js report tool whose 'Pro export' feature is meant to be gated behind a Keysat license.",
+  "scripts": {
+    "dev": "next dev -p 4311",
+    "build": "next build",
+    "start": "next start -p 4311"
+  },
+  "dependencies": {
+    "next": "15.1.6",
+    "react": "19.0.0",
+    "react-dom": "19.0.0"
+  },
+  "devDependencies": {
+    "@types/node": "22.10.7",
+    "@types/react": "19.0.7",
+    "@types/react-dom": "19.0.3",
+    "typescript": "5.7.3"
+  }
+}