fix(tui): don't fallback to first provider when agents loading

When using the placeholder agent (before real agents load), return
undefined instead of falling back to the first provider's model.
This prevents showing google-vertex-anthropic models before the
agent's configured model loads.

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

packages/agent-core/src/cli/cmd/tui/context/local.tsx

@@ -205,15 +205,19 @@ export const { use: useLocal, provider: LocalProvider } = createSimpleContext({
 
       const currentModel = createMemo(() => {
         const a = agent.current()
+        // If using placeholder agent (no name), don't return any model yet
+        if (!a?.name) {
+          return undefined
+        }
         // Agent's configured model takes priority - trust it without validation
         // Custom models (like cerebras/zai-glm-4.7) may not be in the provider list
-        if (a?.model) {
+        if (a.model) {
           return a.model
         }
         // For non-configured agents, use cached selection or fallback (with validation)
         return (
           getFirstValidModel(
-            () => modelStore.model[a?.name ?? ""], // Cached selection
+            () => modelStore.model[a.name], // Cached selection
             fallbackModel, // Global fallback
           ) ?? undefined
         )