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__/GitHubImportModal.test.tsx

@@ -8,7 +8,7 @@ import {
   apiImportGitHubPull,
   fetchGitRemotes,
 } from "../../api";
-import type { Task } from "@kb/core";
+import type { Task } from "@fusion/core";
 import type { GitRemote } from "../../api";
 
 // Mock the API module