Repository `node_id` derivation

When a repository fixture omits node_id, Simulacat Core derives it as base64("Repository:owner/name"). Use repositoryNodeId when code needs the same canonical node identifier without building a full fixture.

import {repositoryNodeId} from 'simulacat-core';

const id = repositoryNodeId({owner: 'acme', name: 'awesome-repo'});
console.log(id);