fix(planner): increase brief maxTokens from 1600 to 4096

The methodology-aware system prompt causes the planner to produce
much richer beat instructions (concrete scene descriptions instead
of abstract labels). 1600 tokens was insufficient — the LLM output
was truncated mid-JSON, causing all 5 bench books to fail.

packages/core/src/agents/planner.ts

@@ -164,7 +164,7 @@ export class PlannerAgent extends BaseAgent {
       },
     ], {
       temperature: 0.2,
-      maxTokens: 1600,
+      maxTokens: 4096,
     });
 
     const parsed = this.tryParseChapterBrief(response.content);