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/stuck-task-detector.ts

@@ -11,7 +11,7 @@
  * compares the last activity timestamp against `taskStuckTimeoutMs` from settings.
  */
 
-import type { TaskStore, Settings } from "@kb/core";
+import type { TaskStore, Settings } from "@fusion/core";
 import { createLogger } from "./logger.js";
 
 const stuckLog = createLogger("stuck-detector");