feat(FN-3716): add planning mode priority controls for task routes and subt

Adds per-subtask priority controls to planning mode with three implementation steps (route priority support, UI controls, and design-token polish), gates research tools behind an experimental feature flag with a new core infrastructure module, and introduces a WhatsApp chat plugin with discovery and

Fusion-Task-Id: FN-3716

packages/core/src/types.ts

@@ -3370,6 +3370,7 @@ export interface PlanningSummary {
   title: string;
   description: string;
   suggestedSize: "S" | "M" | "L";
+  priority?: TaskPriority;
   suggestedDependencies: string[];
   keyDeliverables: string[];
 }