SAM is for simple serverless stacks; this project has ECS, VPC, and
multi-service composition which requires CDK. Migration brings
ContainerImage.fromAsset() for automatic Docker builds on deploy,
matching the seahaven-slack-bot pattern.
Also fixes: Bedrock model ID (add version suffix), Gmail history API
parameter (labelId not labelIds), classify JSON extraction (handle
markdown fences), and digest block limit (cap sections at 5 items
to stay under Slack's 50-block limit).
