feat: add topic hierarchy APIs

tests/queries.test.ts

@@ -19,6 +19,16 @@ async function seedMemoryGraph(db: IdentityDB): Promise<void> {
       { name: 'programming language', category: 'concept', granularity: 'abstract', role: 'classification' },
     ],
   });
+
+  await db.linkTopics({
+    parentName: 'software technology',
+    childName: 'programming language',
+  });
+
+  await db.linkTopics({
+    parentName: 'programming language',
+    childName: 'TypeScript',
+  });
 }
 
 describe('IdentityDB queries', () => {
@@ -60,6 +70,7 @@ describe('IdentityDB queries', () => {
       '2025',
       'I',
       'programming language',
+      'software technology',
       'TypeScript',
     ]);
     expect('facts' in topics[0]!).toBe(false);
@@ -81,4 +92,25 @@ describe('IdentityDB queries', () => {
     expect(facts).toHaveLength(1);
     expect(facts[0]?.statement).toBe('I have worked with TypeScript since 2025.');
   });
+
+  it('lists direct child topics for a parent topic', async () => {
+    const children = await db.getTopicChildren('programming language');
+
+    expect(children.map((topic) => topic.name)).toEqual(['TypeScript']);
+  });
+
+  it('lists direct parent topics for a child topic', async () => {
+    const parents = await db.getTopicParents('TypeScript');
+
+    expect(parents.map((topic) => topic.name)).toEqual(['programming language']);
+  });
+
+  it('returns lineage from nearest parent outward', async () => {
+    const lineage = await db.getTopicLineage('TypeScript');
+
+    expect(lineage.map((topic) => topic.name)).toEqual([
+      'programming language',
+      'software technology',
+    ]);
+  });
 });