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/new-workspace-screen.tsx

@@ -289,8 +289,7 @@ export function NewWorkspaceScreen({
             serverId={serverId}
             isInputActive={true}
             onSubmitMessage={handleCreateChatAgent}
-            allowEmptySubmit
-            emptySubmitLabel="Create"
+            hasExternalContent={selectedGithubItem !== null}
             isSubmitLoading={pendingAction === "chat"}
             blurOnSubmit={true}
             value={chatDraft.text}

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}