feat(open-swe): disable "Request changes" until the plan has a comment (#1602)

Requesting changes hands the reviewer comments to the agent, so it's
meaningless with none. Disable the button (with a hint tooltip) until at
least one comment exists; approve is unaffected.

tests/e2e/tests/plan_review.spec.ts

@@ -89,10 +89,14 @@ test.describe("Plan review (HTTP comments)", () => {
       timeout: 30_000,
     });
     await expect(owner.getByTestId("approve-plan")).toBeVisible();
+    // "Request changes" is meaningless with no feedback → disabled until a
+    // comment exists.
+    await expect(owner.getByTestId("reject-plan")).toBeDisabled();
 
     // Owner leaves a comment.
     await addComment(owner, "Owner: looks solid, ship it.");
     await expect(owner.getByTestId("plan-comment")).toHaveCount(1);
+    await expect(owner.getByTestId("reject-plan")).toBeEnabled();
 
     // 4. A COLLABORATOR opens the same plan: sees it AND the owner's comment
     //    (fetched over HTTP), but has NO approve button.

ui/src/components/agents/PlanReview.tsx

@@ -128,7 +128,14 @@ export function PlanReview({ plan }: { plan: PlanData }) {
           <Button
             data-testid="reject-plan"
             variant="secondary"
-            disabled={busy !== null || decision !== null}
+            // Requesting changes feeds the comments to the agent, so it's
+            // meaningless with none — disable until at least one is left.
+            disabled={busy !== null || decision !== null || comments.length === 0}
+            title={
+              comments.length === 0
+                ? "Leave a comment first to request changes"
+                : undefined
+            }
             onClick={() => void decide("reject")}
           >
             Request changes