Implement CompileUserSchema() []byte in schema/schema.go. It must return a JSON Schema document (the meta-document with "type","properties","required","additionalProperties" — NOT an example value) describing a user object, valid for OpenAI/codex --output-schema (strict structured outputs). The user object has name (string) and address, where address is itself a nested object with street (string) and city (string). All existing tests in schema/schema_test.go must pass.
