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/tsconfig.json

@@ -0,0 +1,21 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "lib": ["dom", "dom.iterable", "esnext"],
+    "allowJs": true,
+    "skipLibCheck": true,
+    "strict": true,
+    "noEmit": true,
+    "esModuleInterop": true,
+    "module": "esnext",
+    "moduleResolution": "bundler",
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "jsx": "preserve",
+    "incremental": true,
+    "plugins": [{ "name": "next" }],
+    "paths": { "@/*": ["./*"] }
+  },
+  "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx", ".next/types/**/*.ts"],
+  "exclude": ["node_modules"]
+}