refactor(products): unify product command registration

[Episkey-G]

Summary

• unify cli.Product.NewCommand(ctx) to return []*cobra.Command, allowing one real product domain to own multiple top-level CLI commands
• consolidate migrated command products into real product domains: eip/firewall under unet, and image under uhost
• refresh product registration, command/completion goldens, onboarding example, and add registration coverage for multi-command products
• normalize empty static completion golden rows to avoid trailing whitespace in newly generated snapshots

Validation

• go test ./cmd -run TestUhost -count=1 -timeout=30m
• go test ./... -skip TestUhost -count=1
• go test ./hack/snapshot -count=1
• go run ./hack/check-product
• go test ./cmd -run 'TestRegisteredProductsUseRealProductDomains|TestAddProductCommandsRegistersAllProductCommands' -count=1
• git diff --cached --check