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/scheduler.ts

@@ -1,4 +1,4 @@
-import { resolveDependencyOrder, type TaskStore, type Task } from "@kb/core";
+import { resolveDependencyOrder, type TaskStore, type Task } from "@fusion/core";
 import type { AgentSemaphore } from "./concurrency.js";
 import { schedulerLog } from "./logger.js";
 import type { PrMonitor } from "./pr-monitor.js";