feat(workshop): predeploy ABCA for Workshop Studio

[ayushtr-aws]

Summary

• replace the Workshop Studio S3 smoke test with a full ABCA CDK deploy
• pin the playground Blueprint and seed the runtime GitHub token through an operator-owned broker role
• create a deterministic participant Cognito user with generated credentials
• destroy ABCA, participant credentials, and CDK bootstrap resources on Workshop Studio delete
• prefer native ARM CodeBuild for the AgentCore image build

Validation

• bash -n manage-workshop-stack.sh
• pinned-commit shallow fetch from GitHub
• Node 22 yarn install --frozen-lockfile
• CLI TypeScript compile and global bgagent link

Do not merge until the Workshop Studio test event completes create, submit-to-PR, denied participant CDK deploy, and delete.