Improve onboarding defaults and issue goal fallback

ui/src/lib/onboarding-goal.ts

@@ -0,0 +1,18 @@
+export function parseOnboardingGoalInput(raw: string): {
+  title: string;
+  description: string | null;
+} {
+  const trimmed = raw.trim();
+  if (!trimmed) {
+    return { title: "", description: null };
+  }
+
+  const [firstLine, ...restLines] = trimmed.split(/\r?\n/);
+  const title = firstLine.trim();
+  const description = restLines.join("\n").trim();
+
+  return {
+    title,
+    description: description.length > 0 ? description : null,
+  };
+}