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__/ScheduleStepsEditor.test.tsx

@@ -1,10 +1,10 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { render, screen, fireEvent } from "@testing-library/react";
 import { ScheduleStepsEditor } from "../ScheduleStepsEditor";
-import type { AutomationStep } from "@kb/core";
+import type { AutomationStep } from "@fusion/core";
 
-// Mock @kb/core
-vi.mock("@kb/core", () => ({}));
+// Mock @fusion/core
+vi.mock("@fusion/core", () => ({}));
 
 // Mock lucide-react
 vi.mock("lucide-react", () => ({