docs(message-list): note smooth-scroll exclusion; point sentinel comment at subsystem doc

Reviewer caught two small gaps after Task 3:
- Effect A's smooth-scroll path on new messages is intentionally NOT
  instrumented with the sentinel (the animation lands asynchronously
  across frames; no intermediate scrollTop is worth pinning to). The
  doc now records this so the reader's intuition matches the code.
- The sentinel-branch comment in MessageList.tsx pointed at "spec §2",
  which is the planning doc rather than the durable subsystem spec.
  Pointed at docs/systems/message-list.md instead.

packages/web/src/components/chat/MessageList.tsx

@@ -240,7 +240,7 @@ export function MessageList({ channelId, jumpToMessageId, onJumpComplete }: Mess
     // was queued by our own command. Layout may have grown between the command and the
     // event firing, but our intent is "stay at bottom" — do not let a post-growth distance
     // measurement flip the at-bottom flags. Re-pin defensively (content may have grown
-    // again) and update the sentinel; convergence is described in the spec §2.
+    // again) and update the sentinel. See docs/systems/message-list.md (Auto-scroll model).
     if (container.scrollTop === lastProgrammaticBottomScrollRef.current) {
       isAtBottomRef.current = true;
       setIsAtBottom(true);