fix(cli): validate UModel YAML files

[Chloride233]

Summary

This closes a small gap in the local authoring loop: the CLI docs point umctl umodel validate at a quickstart UModel YAML file, but the command only wrapped JSON before calling the validate API.

Now umctl umodel validate can accept one schema-style UModel YAML file and send the same elements payload the API already expects.

Scope

• Reuses the UModel payload normalizer from import parsing.
• Keeps umodel put JSON-only.
• No REST API, MCP, SDK, schema, or Web UI changes.

Tests

• go test ./cmd/umctl/cmd -run 'TestUModelValidateAcceptsSchemaStyleYAMLFile|TestCLICommandsRouteToCorrectEndpoints' -count=1
• go test ./internal/umodel/payload -count=1
• go test ./internal/bootstrap ./internal/sampledata -run 'QuickStart|quickstart|Sample|Localization' -count=1
• git diff --check