fix(workshop): bind CDK environment and use AgentCore-supported AZs

## Problem

Workshop Studio ABCA provisioning exposed two environment-specific failures:

1. CDK bootstrap created `/cdk-bootstrap/hnb659fds/version` in the workshop account, but deploy resolved `CDK_DEFAULT_ACCOUNT` / `CDK_DEFAULT_REGION` from a different environment and reported the parameter missing.
2. After explicitly overriding those values, AgentCore Runtime creation rejected a private subnet in zone ID `use1-az6`; AgentCore supports `use1-az4`, `use1-az1`, and `use1-az2` in us-east-1.

## Scope

- Bind create, update, and delete operations to the active AWS caller account and workshop region.
- Resolve supported AgentCore zone IDs to account-local AZ names before synthesis.
- Pass the resolved names through CDK context and constrain the workshop VPC to those zones.
- Preserve default AZ selection outside the workshop path.
- Keep AgentCore Registry disabled for Workshop Studio.
- Add focused construct and stack tests.