feat: serialize 'answer' stub nodes in markdown output

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/treeMarkdownSync.ts

@@ -78,6 +78,10 @@ function serializeNode(
     if (node.resolution_steps?.length) {
       node.resolution_steps.forEach((step, i) => body.push(`${i + 1}. ${step}`))
     }
+  } else if (node.type === 'answer') {
+    // Answer placeholder — render as a clearly marked stub
+    body.push(`## [ANSWER PLACEHOLDER] ${node.title || 'Untitled'}`, '')
+    body.push('> This is an unresolved answer stub. Convert it to a Decision, Action, or Solution before publishing.')
   }
 
   blocks.push(fm.join('\n') + '\n' + body.join('\n'))