Migrate from the localstack CLI to lstk

[HarshCasper]

Summary

• Replace localstack/awslocal/cdklocal invocations with lstk across the Makefile, README, and both CI workflows.
• Replace the setup-localstack and cloud-pods actions in pods.yml with npm install -g @localstack/lstk + lstk start + lstk snapshot load pod:loan-broker-infra.
• Bump the aws-cdk CLI pin from 2.70.0 to 2.1133.0 (lstk requires ≥ 2.177.0); aws-cdk-lib is unchanged.

Testing

• CI workflow run on this branch.

Part of DEVX-1056

[peter-smith-phd]

Tests are failing. I'll review the code once they pass.

[HarshCasper]

Tests are green now. The loan-broker-infra pod was missing from the platform, so the Cloud Pods job could never load it — re-created it (deployed both stacks on a clean emulator, lstk snapshot save pod:loan-broker-infra) and verified the CI path locally: fresh emulator → lstk snapshot load → pytest passes. Both checks pass on the latest run.