fix(l1): answer buttons must match the question — yes_label/no_label end-to-end

Live walk defect: the builder generated alternatives questions ("Is Jane's
account a Microsoft account or a local account?") while the UI could only
offer Yes/No. Root cause: SYSTEM_PROMPT mandated a label-less
'<yes/no question>' shape with no way to express the two answers.

- SYSTEM_PROMPT: question nodes must carry yes_label/no_label — the literal
  button texts; alternatives questions must use the alternatives as labels.
- validate_node: labels hard-floor-scanned, must be distinct non-empty strings.
- _ensure_labels: server defaults missing labels to Yes/No.
- advance_ai_build: records answer_label (and both labels) in walked_path,
  derived from the server-held pending_node — never client-supplied.
- _build_context: LLM context shows the chosen label, not a bare yes/no
  (a raw "-> yes" on an alternatives question degrades the next generation).
- normalize_walked_path: captured flywheel trees keep question labels.
- Frontend: buttons render yes_label/no_label; walk transcript and
  L1EscalationsSection render answer_label.

Phase 2A backend set: 137 passed / 0 failed / 8 deselected. tsc, eslint,
vite build clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/l1/L1WalkTreeVariant.tsx

@@ -167,13 +167,13 @@ export function L1WalkTreeVariant({ session, onSessionUpdate, onDone }: Props) {
                       onClick={() => advanceNode({ answer: 'yes' })}
                       className="flex-1 rounded-md bg-accent text-white py-3 text-base font-medium hover:bg-accent/90 min-h-[44px] transition-colors"
                     >
-                      Yes
+                      {node.yes_label ?? 'Yes'}
                     </button>
                     <button
                       onClick={() => advanceNode({ answer: 'no' })}
                       className="flex-1 rounded-md border border-default py-3 text-base font-medium hover:bg-elevated min-h-[44px] transition-colors"
                     >
-                      No
+                      {node.no_label ?? 'No'}
                     </button>
                   </div>
                 </>
@@ -252,7 +252,7 @@ export function L1WalkTreeVariant({ session, onSessionUpdate, onDone }: Props) {
               {session.walked_path.map((step, i) => (
                 <li key={i} className="flex flex-col">
                   <span className="text-muted-foreground text-xs">{step.question ?? step.text}</span>
-                  {step.answer && <span className="font-medium">→ {step.answer}</span>}
+                  {step.answer && <span className="font-medium">→ {step.answer_label ?? step.answer}</span>}
                   {step.l1_note && <span className="text-muted-foreground text-xs italic mt-0.5">{step.l1_note}</span>}
                 </li>
               ))}