ci: remove the WonderTwin workaround and probe steps from the integration job #24

The integration job in .github/workflows/ci.yml (added in #20) has two temporary steps that should go once LocalStack hosts the app emulators itself:

• "Probe the app emulators": a debugging aid.
• "Run the app emulators LocalStack does not host": builds stripe, plaid and xero from WonderTwin (pinned through WONDERTWIN_REF) and points the demo at them through <APP>_EMULATOR_URL.

Today all three answer 404 on LocalStack, so every emulator in the job comes from WonderTwin and LocalStack is started but not exercised.

The workaround step checks each vendor on LocalStack first and only builds the ones that don't answer, so it will quietly do nothing once LocalStack covers them. It still costs build time and depends on the WonderTwin repo, so it should be removed.

Remove both steps once

• plaid is merged into localstack-ext main and shipped in a release (or the image the sandbox uses). The plugin is on the apps/plaid-ca9fea68d873 branch.
• xero is merged into localstack-ext main and shipped in a release (or the image the sandbox uses). The plugin is on the apps/xero-b24aeed25cff branch.
• the sandbox image is new enough to serve stripe. It runs 2026.8.4 today, and stripe is in current LocalStack.

When removing

• Delete both steps, WONDERTWIN_REF, and the job comment that says the emulators come from WonderTwin.
• Confirm the integration test passes with all three served by LocalStack.