Pluggable composition backends (Crossplane vs KRO+ACK)

**Parent:** #584
**Priority:** Low
**Blocked by:** #600

Allow users to choose between Crossplane and KRO+ACK for infrastructure provisioning, similar to how `cluster-providers/` offers pluggable bootstrap approaches.

**Design:**
- `abstractions/resource-groups/platform-cluster/` becomes the Crossplane backend
- `abstractions/resource-groups/platform-cluster-kro/` becomes the KRO+ACK backend
- `config.local.yaml` gets a `compositionBackend: crossplane | kro` field
- Bootstrap Taskfile selects the correct abstraction based on config
- Both backends produce the same outputs (EKS cluster, VPC, IAM roles)
- `bootstrap/clusters.yaml` ApplicationSet works with either backend

**Acceptance criteria:**
- Both backends can provision and destroy a hub cluster
- Spoke provisioning works with either backend
- Switching backends on a new install requires only a config change