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('acme', 'awesome-repo');
// Buffer.from(id, 'base64').toString() === 'Repository:acme/awesome-repo'