Replace the TODO placeholder for error codes with a proper WrenInterpretResult enum containing WREN_RESULT_SUCCESS, WREN_RESULT_COMPILE_ERROR, and WREN_RESULT_RUNTIME_ERROR. Update wrenInterpret signature to return the new type and refactor callers in main.c and wren_vm.c to use the enum values instead of magic exit codes. Also add a startup banner to the REPL.