feat(gateway): add system.sessionAnalytics usage snapshot RPC

src/session/index.ts

@@ -1,4 +1,5 @@
 export { SessionStore, parseDuration } from './store.js';
+export type { SessionAnalyticsSnapshot, SessionDailyAnalyticsRow, SessionTopAnalyticsRow } from './store.js';
 export { SessionManager, ManagedSession, type Session } from './manager.js';
 export { SessionIndexer, tokenize } from './indexer.js';
 export type { HistoryMetadata, HistoryIndexerConfig } from './indexer.js';