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/engine/src/pr-comment-handler.test.ts

@@ -1,6 +1,6 @@
 import { describe, it, expect, vi, beforeEach } from "vitest";
 import { PrCommentHandler } from "./pr-comment-handler.js";
-import type { TaskStore, Task } from "@kb/core";
+import type { TaskStore, Task } from "@fusion/core";
 
 const mockStore = {
   addSteeringComment: vi.fn<(id: string, text: string, author?: "user" | "agent") => Promise<Task>>(),