fix: natural follow-up conversation flow

- Grader: separate followup hint from scoring feedback
- Interviewer: use followup hint directly without prefix/suffix
- Restored standard and choice question presentation paths

server/src/assessment/graph/nodes/interviewer.node.spec.ts

@@ -81,14 +81,13 @@ describe('interviewerNode', () => {
   });
 
   describe('follow-up mode', () => {
-    it('should include feedback in follow-up prompt', async () => {
+    it('should use last feedbackHistory message content as follow-up prompt', async () => {
       const state = baseState({
         shouldFollowUp: true,
-        feedbackHistory: [new AIMessage('Feedback: You need more details')],
+        feedbackHistory: [new AIMessage('You need more details')],
       });
       const result = await interviewerNode(state);
       const msg = (result.messages as any)[0].content as string;
-      expect(msg).toContain('Follow-up');
       expect(msg).toContain('You need more details');
     });