Set document title on agent screen for web

Adds a useEffect that sets the browser tab title to the agent's title
(or "Agent" as fallback) when viewing an agent. Only runs on web platform.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/app/src/app/agent/[serverId]/[agentId].tsx

@@ -8,6 +8,7 @@ import {
   useWindowDimensions,
   LayoutChangeEvent,
   ScrollView,
+  Platform,
 } from "react-native";
 import { useLocalSearchParams, useRouter } from "expo-router";
 import { useFocusEffect } from "@react-navigation/native";
@@ -357,6 +358,14 @@ function AgentScreenContent({ serverId, agentId, onBack }: AgentScreenContentPro
     initializeAgent({ agentId: resolvedAgentId });
   }, [resolvedAgentId, initializeAgent, isInitializingFromMap]);
 
+  useEffect(() => {
+    if (Platform.OS !== "web") {
+      return;
+    }
+    const title = agent?.title || "Agent";
+    document.title = title;
+  }, [agent?.title]);
+
   const recalculateMenuPosition = useCallback(
     (onMeasured?: () => void) => {
       requestAnimationFrame(() => {