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__/ThemeSelector.test.tsx

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi } from "vitest";
 import { render, screen, fireEvent } from "@testing-library/react";
-import { COLOR_THEMES } from "@kb/core";
+import { COLOR_THEMES } from "@fusion/core";
 import { ThemeSelector } from "../ThemeSelector";
 
 describe("ThemeSelector", () => {