fix(types): make Session.isActive writable and add revokedAt

Required for session revocation fix — close() needs to set isActive=false
and revokedAt timestamp on the Session object.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: ersinkoc

packages/core/src/services/session-service.ts

@@ -45,7 +45,10 @@ export interface Session {
   readonly platformChatId?: string;
 
   /** Whether the session is currently active */
-  readonly isActive: boolean;
+  isActive: boolean;
+
+  /** Timestamp when session was revoked (set by close()) */
+  revokedAt?: Date;
 
   /** Arbitrary session metadata */
   readonly metadata: Record<string, unknown>;