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/__tests__/Board.test.tsx

@@ -2,9 +2,9 @@ import React from "react";
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { render, screen, fireEvent } from "@testing-library/react";
 import { Board } from "../Board";
-import { COLUMNS } from "@kb/core";
+import { COLUMNS } from "@fusion/core";
 
-import type { Task } from "@kb/core";
+import type { Task } from "@fusion/core";
 
 const columnRenderCounts: Record<string, number> = {};