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/utils/worktreeGrouping.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect } from "vitest";
 import { groupByWorktree, getWorktreeLabel } from "./worktreeGrouping";
-import type { Task } from "@kb/core";
+import type { Task } from "@fusion/core";
 
 function makeTask(overrides: Partial<Task> & { id: string }): Task {
   return {