feat: add isContextOnly flag to extraction messages to prevent repeated extraction

- buildExtractionMessages marks messages with isContextOnly (seq < startIdx)
- formatExtractionTranscript inserts section dividers for mixed context/target
- extractor.js Layer 1 prompt splits context review vs extraction target with guidance
- Add tests/extraction-context-only-flag.mjs (7 test cases)

maintenance/chat-history.js

@@ -286,6 +286,7 @@ export function buildExtractionMessages(chat, startIdx, endIdx, settings) {
       rawContent: String(msg?.mes ?? ""),
       name: String(msg?.name ?? "").trim(),
       speaker: String(msg?.name ?? "").trim(),
+      isContextOnly: index < startIdx,
     });
   }