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/screens/workspace/workspace-draft-agent-tab.tsx

@@ -287,7 +287,6 @@ export function WorkspaceDraftAgentTab({
             serverId={serverId}
             isInputActive={isPaneFocused}
             onSubmitMessage={handleCreateFromInput}
-            allowEmptySubmit={false}
             isSubmitLoading={isSubmitting}
             blurOnSubmit={true}
             value={draftInput.text}