Keep new-workspace composer expanded after submit

The new-workspace composer uses preserve-and-lock: it keeps the typed
text through workspace creation so it survives a failure. But the input
minimized its height on every submit, collapsing a multi-line draft to
one line while the text and loading spinner remained — clipped and ugly.

Gate the height-minimize on the same condition that clears the text, so
the box only collapses when the text actually goes away (agent panel),
not when it's preserved (new-workspace).

packages/app/src/composer/index.tsx

@@ -1871,6 +1871,7 @@ export function Composer({
               submitIcon={submitIcon}
               isSubmitDisabled={isSubmitBusy}
               isSubmitLoading={isSubmitBusy}
+              preserveHeightOnSubmit={submitBehavior === "preserve-and-lock"}
               attachments={selectedAttachments}
               cwd={cwd}
               attachmentMenuItems={attachmentMenuItems}