frontend: fix layout shifts on AI agent create page (#2218)

Three sources of visual jank on the create agent page:

1. The MarkdownEditor's TabsContent uses framer-motion with a 500ms
   blur/slide animation. Switching between Editor/Raw tabs triggered
   this jarring transition. Replaced with direct conditional rendering.

2. The MarkdownEditor's editorKey effect fired on initial mount,
   causing Milkdown to unmount and remount immediately. The Prompt
   card visibly jumped as the editor content disappeared and
   reappeared. Now the effect only fires on actual mode changes.

3. The LLM config card grew then shrank as the gateway query
   resolved: API Token field rendered while gateways were loading,
   then disappeared once a gateway was auto-selected. Now the
   gateway-dependent fields (provider, model, API token) show
   skeleton placeholders until the gateway state fully settles.
   Gateway selector and max iterations render immediately.

Author: birdayz

frontend/src/components/pages/agents/create/ai-agent-create-page.tsx

@@ -531,10 +531,10 @@ export const AIAgentCreatePage = () => {
               </Card>
             </div>
 
-            {/* Prompt */}
+            {/* System Prompt */}
             <Card size="full">
               <CardHeader>
-                <CardTitle>Prompt</CardTitle>
+                <CardTitle>System Prompt</CardTitle>
                 <Text variant="muted">Define the agent's behavior and instructions</Text>
               </CardHeader>
               <CardContent>