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/app/page.tsx

@@ -0,0 +1,34 @@
+import { ROWS } from "@/lib/reports";
+
+export default function Home() {
+  return (
+    <main>
+      <h1>Acme Reports</h1>
+      <p>Your signups and revenue by region. Viewing is free.</p>
+      <table cellPadding={6} style={{ borderCollapse: "collapse" }}>
+        <thead>
+          <tr>
+            <th align="left">Region</th>
+            <th align="right">Signups</th>
+            <th align="right">Revenue (sats)</th>
+          </tr>
+        </thead>
+        <tbody>
+          {ROWS.map((r) => (
+            <tr key={r.region}>
+              <td>{r.region}</td>
+              <td align="right">{r.signups}</td>
+              <td align="right">{r.revenueSats.toLocaleString()}</td>
+            </tr>
+          ))}
+        </tbody>
+      </table>
+
+      <h2 style={{ marginTop: "2rem" }}>Pro export</h2>
+      <p>
+        Download the full dataset as CSV. This is a paid feature:{" "}
+        <a href="/api/export">/api/export</a>.
+      </p>
+    </main>
+  );
+}