feat: add missing archive fields to ExperienceSys [DX-981] (#3005)

Add archivedAt, archivedBy, and archivedVersion optional fields to
ExperienceSys to match the upstream ExperienceSysCommon type in
experiences-api-schemas, allowing SDK consumers to read archive state.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: tylerpina

lib/entities/experience.ts

@@ -35,6 +35,9 @@ export type ExperienceSys = {
   updatedAt: string
   createdBy: Link<'User'>
   updatedBy: Link<'User'>
+  archivedAt?: string
+  archivedBy?: Link<'User'>
+  archivedVersion?: number
   variant?: string
   variantType?: string
   variantDimension?: string