feat(cli): add scrollable message history with PgUp/PgDn (TUI-010)

- Create use-viewport hook tracking scroll offset, viewport size, auto-follow
- Update MessageList to slice visible messages and show scroll indicator
- Wire PgUp/PgDn keybindings in app.tsx

packages/cli/src/tui/hooks/use-viewport.ts

@@ -0,0 +1,80 @@
+import { useState, useCallback, useEffect } from 'react';
+import { useStdout } from 'ink';
+
+export interface UseViewportOptions {
+  totalItems: number;
+  reservedLines?: number;
+}
+
+export interface UseViewportReturn {
+  scrollOffset: number;
+  viewportSize: number;
+  isScrolledUp: boolean;
+  scrollToBottom: () => void;
+  scrollBy: (delta: number) => void;
+  scrollTo: (offset: number) => void;
+  canScrollUp: boolean;
+  canScrollDown: boolean;
+}
+
+export function useViewport({
+  totalItems,
+  reservedLines = 10,
+}: UseViewportOptions): UseViewportReturn {
+  const { stdout } = useStdout();
+  const rows = stdout?.rows ?? 24;
+  const viewportSize = Math.max(1, rows - reservedLines);
+
+  const [scrollOffset, setScrollOffset] = useState(0);
+  const [autoFollow, setAutoFollow] = useState(true);
+
+  // Compute the maximum valid scroll offset
+  const maxOffset = Math.max(0, totalItems - viewportSize);
+
+  // Auto-follow: when new items arrive and auto-follow is on, snap to bottom
+  useEffect(() => {
+    if (autoFollow) {
+      setScrollOffset(maxOffset);
+    }
+  }, [autoFollow, maxOffset]);
+
+  const scrollTo = useCallback(
+    (offset: number) => {
+      const clamped = Math.max(0, Math.min(offset, maxOffset));
+      setScrollOffset(clamped);
+      setAutoFollow(clamped >= maxOffset);
+    },
+    [maxOffset],
+  );
+
+  const scrollBy = useCallback(
+    (delta: number) => {
+      setScrollOffset((prev) => {
+        const next = Math.max(0, Math.min(prev + delta, maxOffset));
+        setAutoFollow(next >= maxOffset);
+        return next;
+      });
+    },
+    [maxOffset],
+  );
+
+  const scrollToBottom = useCallback(() => {
+    setScrollOffset(maxOffset);
+    setAutoFollow(true);
+  }, [maxOffset]);
+
+  const isScrolledUp = scrollOffset < maxOffset;
+  const canScrollUp = scrollOffset > 0;
+  const canScrollDown = scrollOffset < maxOffset;
+
+  return {
+    scrollOffset,
+    viewportSize,
+    isScrolledUp,
+    scrollToBottom,
+    scrollBy,
+    scrollTo,
+    canScrollUp,
+    canScrollDown,
+  };
+}