Create PlatformCluster abstraction using KRO + ACK

[punkwalker]

Parent: #584
Priority: Medium

Create an alternative PlatformCluster implementation using KRO ResourceGroups and ACK controllers instead of Crossplane. The existing kro/resource-groups/manifests/eks/ has rg-eks.yaml and rg-eks-vpc.yaml as a starting point.

Scope:

• VPC + subnets + NAT + IGW + route tables (ACK EC2 controller)
• EKS cluster with Auto Mode (ACK EKS controller)
• IAM roles for cluster and nodes (ACK IAM controller)
• Conditional managed node group support
• ArgoCD Capability creation (if ACK EKS supports it, otherwise keep Job)
• Pod identity associations for providers

Advantages over Crossplane:

• AWS-native controllers (first-party, no Upbound dependency)
• Simpler orchestration (KRO ResourceGroups vs Crossplane Compositions + pipeline functions)
• No provider DRC/CRD ownership issues
• ACK uses Pod Identity natively

Challenges to investigate:

• ACK EKS controller support for Auto Mode and ArgoCD Capability
• KRO maturity for complex multi-resource orchestration
• VPC networking dependency ordering in KRO
• Migration path for existing Crossplane-provisioned clusters