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/app.tsx

@@ -6,6 +6,7 @@ import { MessageList } from './components/message-list.js';
 import { InputBar } from './components/input-bar.js';
 import { useSocket } from './hooks/use-socket.js';
 import { useGitInfo } from './hooks/use-git-info.js';
+import { useViewport } from './hooks/use-viewport.js';
 
 export interface TuiAppProps {
   gatewayUrl: string;
@@ -23,10 +24,19 @@ export function TuiApp({ gatewayUrl, conversationId, sessionCookie }: TuiAppProp
     initialConversationId: conversationId,
   });
 
+  const viewport = useViewport({ totalItems: socket.messages.length });
+
   useInput((ch, key) => {
     if (key.ctrl && ch === 'c') {
       exit();
     }
+    // Page Up / Page Down: scroll message history
+    if (key.pageUp) {
+      viewport.scrollBy(-viewport.viewportSize);
+    }
+    if (key.pageDown) {
+      viewport.scrollBy(viewport.viewportSize);
+    }
     // Ctrl+T: cycle thinking level
     if (key.ctrl && ch === 't') {
       const levels = socket.availableThinkingLevels;
@@ -61,6 +71,9 @@ export function TuiApp({ gatewayUrl, conversationId, sessionCookie }: TuiAppProp
         currentStreamText={socket.currentStreamText}
         currentThinkingText={socket.currentThinkingText}
         activeToolCalls={socket.activeToolCalls}
+        scrollOffset={viewport.scrollOffset}
+        viewportSize={viewport.viewportSize}
+        isScrolledUp={viewport.isScrolledUp}
       />
 
       <InputBar

packages/cli/src/tui/components/message-list.tsx

@@ -9,6 +9,9 @@ export interface MessageListProps {
   currentStreamText: string;
   currentThinkingText: string;
   activeToolCalls: ToolCall[];
+  scrollOffset?: number;
+  viewportSize?: number;
+  isScrolledUp?: boolean;
 }
 
 function formatTime(date: Date): string {
@@ -68,17 +71,32 @@ export function MessageList({
   currentStreamText,
   currentThinkingText,
   activeToolCalls,
+  scrollOffset,
+  viewportSize,
+  isScrolledUp,
 }: MessageListProps) {
+  const useSlicing = scrollOffset != null && viewportSize != null;
+  const visibleMessages = useSlicing
+    ? messages.slice(scrollOffset, scrollOffset + viewportSize)
+    : messages;
+  const hiddenAbove = useSlicing ? scrollOffset : 0;
+
   return (
     <Box flexDirection="column" flexGrow={1} paddingX={1}>
+      {isScrolledUp && hiddenAbove > 0 && (
+        <Box justifyContent="center">
+          <Text dimColor>↑ {hiddenAbove} more messages ↑</Text>
+        </Box>
+      )}
+
       {messages.length === 0 && !isStreaming && (
         <Box justifyContent="center" marginY={1}>
           <Text dimColor>No messages yet. Type below to start a conversation.</Text>
         </Box>
       )}
 
-      {messages.map((msg, i) => (
-        <MessageBubble key={i} msg={msg} />
+      {visibleMessages.map((msg, i) => (
+        <MessageBubble key={hiddenAbove + i} msg={msg} />
       ))}
 
       {/* Active thinking */}

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,
+  };
+}