perf: compress chapter context to last 1 chapter + summaries

loadRecentChapters now loads only the most recent chapter (.slice(-1))
instead of the last 3. Earlier chapters are covered by
chapter_summaries.md which is already in context.

packages/core/src/agents/writer.ts

@@ -358,7 +358,7 @@ ${params.volumeOutline}
       const mdFiles = files
         .filter((f) => f.endsWith(".md") && !f.startsWith("index"))
         .sort()
-        .slice(-3);
+        .slice(-1);
 
       if (mdFiles.length === 0) return "";