Migrate from the localstack CLI to lstk

[HarshCasper]

Summary

• Replace localstack/awslocal/cdklocal invocations with lstk across the Makefile, README, and CI workflow.
• Replace the setup-localstack action with npm install -g @localstack/lstk aws-cdk + lstk setup aws + lstk start.
• Unset AWS_ENDPOINT_URL in the Makefile's start/stop/logs recipes; the Makefile exports it globally and lstk lifecycle commands refuse it.
• Remove the unused aws-cdk-local devDependency.

Testing

• CI workflow run on this branch.

Part of DEVX-1056

Note: the failing "CircleCI Pipeline" check is a stale CircleCI integration — the repo has no .circleci/config.yml. Pre-existing; the integration can only be removed in repo/org settings.