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/.gitignore

@@ -0,0 +1,5 @@
+node_modules/
+.next/
+next-env.d.ts
+*.tsbuildinfo
+.env*.local