docs: align the v14 -> v15 migration guides with the breaking-changes audit

AGENTS.md

@@ -125,7 +125,7 @@ Each is a `StateStore`. Components read them with `useStateStore(store, selector
 
 `Channel` takes a single `channel` prop (`src/components/Channel/Channel.tsx`). It activates the channel on mount, deactivates it on unmount, seeds the paginator's unread snapshot, and re-queries on `user.deleted` — nothing else. It does not query the channel for you: initialize it first (`channel.ensureWatched()` sends one watch for overlapping calls, and none for a channel already watched). There is no `Window` component in v15; compose layout yourself or through `ChatView` slots.
 
-Read the bound channel with `useChannel()` (`src/context/useChannel.ts`, thread's channel first, then the `Channel` subtree's) and the active message list with `useMessagePaginator()` (`src/hooks/useMessagePaginator.ts`, thread first, then channel).
+Read the bound channel with `useChannel()` (`src/context/useChannel.ts`, thread's channel first, then the `Channel` subtree's) and the active message list with `useMessagePaginator()` (`src/hooks/useMessagePaginator.ts`, thread first, then channel). `useMessagePaginator()` is internal and not exported from the package; integrator-facing docs and examples read `(thread ?? channel).messagePaginator` via `useThreadContext()` / `useChannel()`.
 
 ### Context layers (`src/context/`)