fix(agent-builder): don't clobber unsent composer text on prefill

A seeded prompt now only populates the composer when it's empty; if the
user has already typed an unsent message, it's preserved (the composer is
still focused). Keeps the prefill genuinely non-destructive.

Addresses Greptile review feedback.

Generated-By: PostHog Code
Task-Id: fc2c5587-1464-4cb1-9116-0e7a34f70734

Author: dmarticus

packages/ui/src/features/agent-applications/components/AgentChatSurface.tsx

@@ -124,12 +124,14 @@ function Composer({
 
   // Prefill (but don't send) when a new draft lands — a seeded prompt drops into
   // the composer for the user to review, edit, and send. Keyed on `token` so the
-  // same prompt re-applies on a fresh trigger.
+  // same prompt re-applies on a fresh trigger. Never clobber text the user has
+  // already typed but not sent: if the composer is non-empty, leave it as-is
+  // (still focusing it) so a seeded prompt is genuinely non-destructive.
   const lastDraftToken = useRef<number | null>(null);
   useEffect(() => {
     if (!draft || draft.token === lastDraftToken.current) return;
     lastDraftToken.current = draft.token;
-    setText(draft.text);
+    setText((current) => (current.trim() ? current : draft.text));
     textareaRef.current?.focus();
   }, [draft]);