swarm-ui try-out: stable live region, reasons for disabled buttons

The running line is a live region whose text changed every second, so a
screen reader announced the counter again each time. The sentence is now
stable and the elapsed seconds sit outside it.

The run and open-a-PR buttons point at the hint that says why they are
off, and add-a-classification says why it is off at the cap.

frontend/packages/swarm-ui/src/ui/button/Button.tsx

@@ -14,12 +14,15 @@ export function Button({
   type = "button",
   disabled,
   onClick,
+  describedBy,
   children,
 }: {
   variant?: ButtonVariant;
   type?: "button" | "submit";
   disabled?: boolean;
   onClick?: (e: Event) => void;
+  /** Id of the element that says why the button is disabled. */
+  describedBy?: string;
   children: ComponentChildren;
 }) {
   return (
@@ -28,6 +31,7 @@ export function Button({
       type={type}
       disabled={disabled}
       onClick={onClick}
+      aria-describedby={describedBy}
     >
       {children}
     </button>