refactor: rename getNewMessages to consumeNewMessages

Rename to signal mutation behavior - the function advances the cursor
as a side effect, so 'consume' better reflects that calling it twice
with the same input yields different results.

src/shared/session-cursor.ts

@@ -40,7 +40,7 @@ function buildMessageKey(message: CursorMessage, index: number): string {
   return `i:${index}`
 }
 
-export function getNewMessages<T extends CursorMessage>(
+export function consumeNewMessages<T extends CursorMessage>(
   sessionID: string | undefined,
   messages: T[]
 ): T[] {