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/core/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@kb/core",
+  "name": "@fusion/core",
   "version": "0.1.0",
   "type": "module",
   "exports": {