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

.changeset/rename-internal-packages.md

@@ -0,0 +1,8 @@
+---
+"@dustinbyrne/kb": patch
+"@fusion/core": patch
+"@fusion/dashboard": patch
+"@fusion/engine": patch
+---
+
+Rename internal packages from @kb/* to @fusion/* namespace