fix: use neutral styling for delete worktree button on merged PRs

The delete worktree button was styled as destructive (red) even though
deleting a worktree for a merged PR is routine cleanup, not a dangerous action.

src/renderer/src/components/right-sidebar/PRActions.tsx

@@ -126,7 +126,7 @@ export default function PRActions({
       <button
         className={cn(
           'w-full flex items-center justify-center gap-1.5 rounded-md px-3 py-1.5 text-[11px] font-medium transition-colors',
-          'bg-destructive text-destructive-foreground hover:bg-destructive/90'
+          'bg-secondary text-secondary-foreground hover:bg-secondary/80'
         )}
         onClick={handleDeleteWorktree}
       >