feat(KB-330): rename internal packages from @kb/* to @fusion/*

- Rename @kb/core, @kb/dashboard, @kb/engine to @fusion/* namespace
- Update all import statements across 143+ files to use new package names
- Update workspace dependencies and root package.json references
- Fix bundler configurations (tsup, vite) for new package names
- Update test files and fix typecheck issues
- Add changeset file documenting the package rename

packages/dashboard/app/components/PlanningModeModal.tsx

@@ -1,5 +1,5 @@
 import { useState, useCallback, useEffect, useRef } from "react";
-import type { Task, PlanningQuestion, PlanningSummary } from "@kb/core";
+import type { Task, PlanningQuestion, PlanningSummary } from "@fusion/core";
 import {
   startPlanning,
   startPlanningStreaming,