fix: back button returns to parent agent instead of homepage

When viewing a child agent (an agent with parentAgentId set), the back
button now navigates to the parent agent screen instead of always going
to the homepage. Added handleBack callback in AgentScreenContent that
checks for parentAgentId and routes accordingly.

🤖 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

@@ -153,6 +153,26 @@ function AgentScreenContent({ serverId, agentId, onBack }: AgentScreenContentPro
     resolvedAgentId ? state.sessions[serverId]?.agents?.get(resolvedAgentId) : undefined
   );
 
+  // Get parent agent ID for back navigation
+  const parentAgentId = agent?.parentAgentId;
+
+  // Navigate to parent agent if this is a child, otherwise go to homepage
+  const handleBack = useCallback(() => {
+    if (parentAgentId) {
+      // Child agent: navigate back to parent
+      router.push({
+        pathname: "/agent/[serverId]/[agentId]",
+        params: {
+          serverId: serverId,
+          agentId: parentAgentId,
+        },
+      });
+    } else {
+      // Root agent: navigate to homepage
+      onBack();
+    }
+  }, [parentAgentId, router, serverId, onBack]);
+
   // Select the agents Map directly - this is a stable reference that only changes when agents are added/removed
   const allAgents = useSessionStore((state) => state.sessions[serverId]?.agents);
 
@@ -507,7 +527,7 @@ function AgentScreenContent({ serverId, agentId, onBack }: AgentScreenContentPro
         {/* Header */}
         <BackHeader
           title={agent.title || "Agent"}
-          onBack={onBack}
+          onBack={handleBack}
           rightContent={
             <View ref={menuButtonRef} collapsable={false}>
               <Pressable onPress={handleOpenMenu} style={styles.menuButton}>