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/merger.test.ts

@@ -32,7 +32,7 @@ import {
 } from "./merger.js";
 import { createKbAgent } from "./pi.js";
 import { execSync } from "node:child_process";
-import { type TaskStore, type Task, type MergeResult, DEFAULT_SETTINGS } from "@kb/core";
+import { type TaskStore, type Task, type MergeResult, DEFAULT_SETTINGS } from "@fusion/core";
 
 const mockedCreateHaiAgent = vi.mocked(createKbAgent);
 const mockedExecSync = vi.mocked(execSync);