fix(dialtone-docs): DLT-3109 address PR review feedback

- Fix non-deterministic date serialization: String(Date) is
  timezone-dependent, use toISOString().split('T')[0] for stable
  YYYY-MM-DD output
- Remove redundant null assertion in type field test

Author: belumontoya

packages/dialtone-docs/src/generators/build-ai-docs.mjs

@@ -49,7 +49,9 @@ function buildEntry(absolutePath) {
     keywords: Array.isArray(frontmatter.keywords) ? frontmatter.keywords : [],
     summary: frontmatter.ai_summary ?? null,
     content,
-    lastUpdated: frontmatter.last_updated ? String(frontmatter.last_updated) : null,
+    lastUpdated: frontmatter.last_updated instanceof Date
+      ? frontmatter.last_updated.toISOString().split('T')[0]
+      : (frontmatter.last_updated ?? null),
     relatedPackages: Array.isArray(frontmatter.related_packages) ? frontmatter.related_packages : [],
     filePath,
   };

packages/dialtone-docs/tests/tests/build-output.test.js

@@ -43,7 +43,6 @@ describe('Build output — dist/ai-docs.json', () => {
 
   test('type field uses allowed values', () => {
     for (const doc of docs) {
-      expect(doc.type, `"${doc.id}" type is null`).not.toBeNull();
       expect(ALLOWED_TYPES, `"${doc.id}" invalid type "${doc.type}"`).toContain(doc.type);
     }
   });