feat(qmd): add shared Node helpers and index.yml model block editing

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/tests/fixtures/yaml-shim.mjs

@@ -0,0 +1,7 @@
+// Loads QMD's bundled yaml package for tests.
+import { qmdPackageDir } from '../../../shared/qmd/qmd-lib.mjs';
+import { pathToFileURL } from 'node:url';
+import { join } from 'node:path';
+const YAML = await import(pathToFileURL(join(qmdPackageDir(), 'node_modules', 'yaml', 'dist', 'index.js')).href);
+export const parseDocument = YAML.parseDocument;
+export default YAML;