fix: use useShallow for childAgents selector to prevent infinite loop

The childAgents selector in the agent screen was creating a new array
on every render, causing an infinite loop when combined with zustand's
store subscription. Added useShallow wrapper to perform shallow
comparison and prevent unnecessary re-renders.

Also updated plan.md with test results:
- PARTIAL PASS: Fixed infinite loop, agent screen loads correctly
- Sub-Agents menu section displays properly
- Parent/child MCP creation works but parentAgentId not auto-injected
- Added fix task for auto-injecting parentAgentId in MCP server

🤖 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

@@ -1,4 +1,5 @@
 import { useEffect, useMemo, useRef, useCallback, useState } from "react";
+import { useShallow } from "zustand/shallow";
 import {
   View,
   Text,
@@ -154,18 +155,21 @@ function AgentScreenContent({ serverId, agentId, onBack }: AgentScreenContentPro
   );
 
   // Select child agents (agents where parentAgentId === current agentId)
-  const childAgents = useSessionStore((state) => {
-    if (!resolvedAgentId) return [];
-    const agents = state.sessions[serverId]?.agents;
-    if (!agents) return [];
-    const children: Array<{ id: string; title: string | null }> = [];
-    for (const [id, a] of agents) {
-      if (a.parentAgentId === resolvedAgentId) {
-        children.push({ id, title: a.title });
+  // Use useShallow to avoid creating new array references on every render
+  const childAgents = useSessionStore(
+    useShallow((state) => {
+      if (!resolvedAgentId) return [];
+      const agents = state.sessions[serverId]?.agents;
+      if (!agents) return [];
+      const children: Array<{ id: string; title: string | null }> = [];
+      for (const [id, a] of agents) {
+        if (a.parentAgentId === resolvedAgentId) {
+          children.push({ id, title: a.title });
+        }
       }
-    }
-    return children;
-  });
+      return children;
+    })
+  );
 
   // Select only the specific stream state - use stable empty array to avoid infinite loop
   const streamItemsRaw = useSessionStore((state) =>