chore(client): drop dead UI code

Nothing imports SessionCard, CardSkeleton, shortModel or agentPillLabel —
the Sessions page renders its own list, formatModelName replaced shortModel,
and the UI labels agents through agentOriginLabel. Removes the symbols, their
Public surface entries, and the ARCHITECTURE mention of SessionCard.

client/src/components/Skeleton.tsx

@@ -29,7 +29,6 @@
  * - `StatValueSkeleton` — exported API; see TSDoc on the symbol for behavior.
  * - `TextSkeleton` — exported API; see TSDoc on the symbol for behavior.
  * - `TableRowSkeleton` — exported API; see TSDoc on the symbol for behavior.
- * - `CardSkeleton` — exported API; see TSDoc on the symbol for behavior.
  *
  * ## Testing pointers
  * - Prefer colocated `__tests__` with Vitest + Testing Library for UI.
@@ -63,11 +62,6 @@
  *   the signature and return type as stable unless release notes say otherwise.
  *   When behavior changes, update the `@file` overview and relevant tests.
  *
- * **CardSkeleton**
- *   Part of this module's public contract. Downstream imports should treat
- *   the signature and return type as stable unless release notes say otherwise.
- *   When behavior changes, update the `@file` overview and relevant tests.
- *
  * ----------------------------------------------------------------------------- */
 
 import type { CSSProperties } from "react";
@@ -145,19 +139,3 @@ export function TableRowSkeleton({ columns, widths }: { columns: number; widths?
     </tr>
   );
 }
-
-/**
- * Block placeholder for card-shaped content (kanban cards, list items).
- */
-export function CardSkeleton({ className = "" }: { className?: string }) {
-  return (
-    <div className={`card p-4 space-y-3 ${className}`} aria-busy="true" aria-label="Loading">
-      <Skeleton className="h-4 w-3/4" />
-      <Skeleton className="h-3 w-1/2" />
-      <div className="flex gap-2">
-        <Skeleton className="h-3 w-12" />
-        <Skeleton className="h-3 w-16" />
-      </div>
-    </div>
-  );
-}