refactor(rete): use asEntityId for AGG_FACT sentinel id

Drop the `-1 as unknown as EntityId` double-cast in favour of the
asEntityId() helper exported alongside the type. Single-site, documented
cast instead of an inline double-cast.

Completes the cleanup of `as unknown as` in non-test source across both
packages/chess and packages/rete.

packages/rete/src/aggregate.ts

@@ -62,7 +62,7 @@
  * ExistentialNode upstream.
  */
 import { Token, type Bindings } from "./beta.js";
-import type { EntityId } from "./schema.js";
+import { asEntityId } from "./schema.js";
 import type { AttrKey, FactValue } from "./wm.js";
 
 /** Supported aggregator kinds. Extending this list requires a new case in
@@ -85,7 +85,7 @@ type DeactivateListener = (token: Token) => void;
  * `schema.ts`), so there is no risk of collision.
  */
 const AGG_FACT = {
-  id: -1 as unknown as EntityId,
+  id: asEntityId(-1),
   attr: "__agg__" as AttrKey,
   value: null as FactValue,
 } as const;