Announcement: sam deploy and sam sync now support CloudFormation Express mode

## SAM CLI now supports CloudFormation Express mode

Starting with **SAM CLI v1.163.0**, `sam deploy` and `sam sync` support the new [CloudFormation Express mode](https://aws.amazon.com/about-aws/whats-new/2026/06/aws-cloudformation-cdk/) via the `--express` flag.

### Default behavior

Currently, `--express` defaults to **off** for both `sam deploy` and `sam sync` to maintain backwards compatibility.

⚠️ **Starting September 1, 2026**, we will flip the default for `sam sync` to use express mode by default (`--express` on), since sync is designed for fast development iteration. You can opt out with `--no-express`. The `sam deploy` default will remain off.

### What is Express mode?

Express mode completes stack operations **up to 4x faster** by returning once resource configuration is applied, without waiting for full stabilization (traffic readiness, edge propagation, resource cleanup). Resources continue becoming ready in the background.

### How to use it

```bash
# Deploy with express mode
sam deploy --express --stack-name my-stack --resolve-s3 --capabilities CAPABILITY_IAM

# Sync with express mode
sam sync --express --stack-name my-dev-stack

# Persist in samconfig.toml
sam deploy --express --save-params
```

### Behavior

- `sam deploy --express`: Passes `DeploymentConfig={"Mode": "EXPRESS"}` to CloudFormation's CreateChangeSet API
- `sam sync --express`: Passes `DeploymentConfig={"Mode": "EXPRESS"}` to CreateStack/UpdateStack APIs
- The `--disable-rollback` flag works with express mode (passed inside DeploymentConfig)
- After deployment, a message reminds users that resources may still be stabilizing in the background
- Persists to `samconfig.toml` via `--save-params` as `express = true`

### Requirements

- SAM CLI >= 1.163.0
- No template changes required — works with all existing templates and nested stacks
- Available in all AWS regions where CloudFormation is supported

### Links

- [CloudFormation Express mode announcement](https://aws.amazon.com/about-aws/whats-new/2026/06/aws-cloudformation-cdk/)
- [PR #9106](https://github.com/aws/aws-sam-cli/pull/9106)

The issue names no files or tests. Start by reviewing PR #9106 and the documented `sam deploy` and `sam sync` behavior; the work is complete when the announcement accurately reflects the released Express mode support, defaults, persistence, and requirements.

Written by the indexing model from the issue text.