fix: require content to create workspace — no more empty submissions

Replace allowEmptySubmit/emptySubmitLabel with hasExternalContent prop
that treats GitHub items the same as text and images in submit gating.

packages/app/src/components/agent-input-submit.ts

@@ -3,7 +3,7 @@ export type AgentInputSubmitResult = "noop" | "queued" | "submitted" | "failed";
 export interface AgentInputSubmitActionInput<TImage> {
   message: string;
   imageAttachments?: TImage[];
-  allowEmptySubmit?: boolean;
+  hasExternalContent?: boolean;
   forceSend?: boolean;
   isAgentRunning: boolean;
   canSubmit: boolean;
@@ -23,7 +23,7 @@ export async function submitAgentInput<TImage>(
   const trimmedMessage = input.message.trim();
   const imageAttachments = input.imageAttachments;
 
-  if (!trimmedMessage && !imageAttachments?.length && !input.allowEmptySubmit) {
+  if (!trimmedMessage && !imageAttachments?.length && !input.hasExternalContent) {
     return "noop";
   }