fix(citations): use all sub-question sources for citation lookup

LLMs may cite chunks from one sub-question's context inside another sub-question's answer section. Previously, processCitationsForSubq only looked up the current sub-question's sources, leaving cross-referenced citations unlinked. Now SubQuestionSection passes all sub-question sources and uses processCitations with a combined flat lookup.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/components/ResponsePanel.tsx

@@ -70,13 +70,18 @@ function SubQuestionSection({
   index,
   subQuestion,
   answerSection,
+  allSubQuestionSources,
 }: {
   index: number
   subQuestion: SubQuestionSources
   answerSection: string
+  allSubQuestionSources: SubQuestionSources[]
 }) {
   const [expanded, setExpanded] = useState(false)
-  const processedAnswer = processCitationsForSubq(answerSection, [subQuestion], 0)
+  // Look up citations across ALL sub-questions' sources because the LLM
+  // may cite chunks from other sub-questions' contexts.
+  const allSources = allSubQuestionSources.flatMap(sq => sq.sources)
+  const processedAnswer = processCitations(answerSection, allSources)
 
   return (
     <div
@@ -211,6 +216,7 @@ function SubQuestionSections({
             index={index}
             subQuestion={subQuestion}
             answerSection={sections[index] ?? ''}
+            allSubQuestionSources={subQuestionSources}
           />
         ))}
       </div>