feat: support CloudFormation Express mode for stages

[hoegertn]

This pull request was created by @kiro-agent on behalf of @hoegertn 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Summary

Implements support for CloudFormation Express mode on a per-stage basis.

Changes

• Added cfnExpressMode optional boolean property to StageOptions interface
• When enabled, the --express flag is added to cdk deploy commands
• Works for all stage types: pipeline stages, independent stages, personal stages, and feature stages

Usage

stages: [
  {
    name: 'dev',
    env: { account: '123456789012', region: 'us-east-1' },
    cfnExpressMode: true, // Enables Express mode for this stage
  },
  {
    name: 'prod',
    env: { account: '987654321098', region: 'us-east-1' },
    // prod uses standard deployment (no Express mode)
  },
],

Testing

• jsii compilation passes with 0 errors, 0 warnings
• TypeScript type checking passes cleanly

Closes #199