- Updated PlatformDbContext: removed GwTenant/GwServiceInstance DbSets, added GwCluster with EF Core config
- Created IClusterStore interface with CRUD and Destination management methods
- Created ClusterStore<TContext> implementation with soft delete and embedded Destinations support
- Deleted obsolete IInstanceStore and InstanceStore (replaced by IClusterStore)
- Updated Extensions.cs and GatewayExtensions.cs to register IClusterStore
Plan 03 of Phase 03 complete.
