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>

backend/app/services/l1_session_service.py

@@ -183,6 +183,24 @@ async def advance_ai_build(
             "answer": answer,
             "l1_note": note,
         }
+        # answer_label: the button text the tech actually clicked. Derived from
+        # the server-held pending_node (never client-supplied) so an
+        # alternatives question ("Microsoft account or local account?") records
+        # "Microsoft account", not a bare "yes", in the transcript, the LLM
+        # context, and the captured flywheel tree.
+        pending = session.pending_node
+        if (
+            answer in ("yes", "no")
+            and isinstance(pending, dict)
+            and pending.get("id") == node_id
+        ):
+            label = pending.get(f"{answer}_label")
+            if label:
+                entry["answer_label"] = label
+            if pending.get("yes_label"):
+                entry["yes_label"] = pending["yes_label"]
+            if pending.get("no_label"):
+                entry["no_label"] = pending["no_label"]
         # JSONB requires assigning a new list — in-place mutation isn't tracked
         session.walked_path = [*session.walked_path, entry]
         session.pending_node = None  # the served node has now been answered