Previously, regions were deleted sequentially even when there were no
cross-region dependencies. Now uses errgroup to delete all regions
concurrently.

- Add cdk, cdk-app, cdk-context inputs to action.yml
- Add CDK mode section to GitHub Actions README docs

- Use npx cdk instead of cdk for synthesis (no global install required)
- Add -i and -s mutual exclusion validation to cdk subcommand
- Fix action.yml stacks option position (delstack cdk $stacks before $cdk_app)
- Refactor cdk.go: extract deletion logic to cdk_deleter.go
- Restructure README CDK section: usage at top, details below
- Add RETAIN_MODE support to cross-region e2e
- Add crossRegionReferences to cross-region e2e CDK app
- Add testgen/e2e retain targets for cdk_integration and cdk_cross_region
- Update CONTRIBUTING.md with all new e2e targets
- Update e2e READMEs with RETAIN documentation

- ParseManifest now recursively parses cdk:cloud-assembly artifacts
  to discover stacks inside CDK Stages (assembly-*/ subdirectories)
- Add unit tests for nested assembly and multiple stages
- Add e2e/cdk_stage/ test environment (2 stages, 2 regions)
- Update Makefile, CONTRIBUTING.md with cdk_stage targets

…CDK Stages

When CDK Stages are used, displayName contains paths like "MyStage/StackA"
which are invalid CloudFormation stack names. The actual stack name is in
properties.stackName.

Stack name resolution priority: properties.stackName > displayName > artifact key

Also fix deploy.go for Stage e2e to use stage glob patterns instead of --all.

- Add all CDK e2e targets to testgen_help and e2e_help in Makefile
- Restructure README: CDK usage at top (How to use), details below
  (CDK Integration Details section with cross-region and Stage docs)
- Fix Stage e2e deploy.go to use stage glob patterns for cdk deploy

…crossRegionReferences)

Pattern A: AddDependency + CFn Export/Import (EdgeStackA -> MainStackA)
Pattern B: crossRegionReferences with SSM (EdgeStackB -> MainStackB)

4 stacks total across 2 regions, testing both CDK cross-region mechanisms.

…lue) + README restructure

- Pattern A now uses AddDependency only (Fn::ImportValue is not supported cross-region)
- Move CDK Integration into How to use as ### subsection
- Remove CDK Stage support section from README (implementation detail)
- Add missing CDK examples to Makefile testgen_help and e2e_help

Same-level stacks across different regions are independent and can be
deleted concurrently. Use errgroup instead of sequential for loop.

Pattern A: EdgeStackA in ap-northeast-1, MainStackA in us-east-1
Pattern B: EdgeStackB in us-east-1, MainStackB in ap-northeast-1

This ensures same-level stacks span both regions, exercising the
errgroup-based parallel region deletion within each dependency level.

Replace level-by-level loop with channel-based dynamic scheduling
(same pattern as existing deleteStacksDynamically). Stacks are started
immediately when their dependencies are satisfied, without waiting for
the entire level to complete.

Example: MainStackA done → EdgeStackA starts immediately, even if
MainStackB is still running.

Skip stacks that don't exist in AWS instead of failing with an error.
This handles the case where cdk.out contains stacks that haven't been
deployed yet.

- Add StackExists method to CloudFormationStackOperator (via DescribeStacks)
- Add filterExistingStacks to cdk.go (operator-based, per-region)
- Add unit tests for StackExists (exists, not found, error)