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/logger.ts

@@ -1,5 +1,5 @@
 /**
- * Lightweight structured logger for the `@kb/engine` package.
+ * Lightweight structured logger for the `@fusion/engine` package.
  *
  * Usage:
  * ```ts