feat(19-02): create MessageCard component for agent inbox

MessageCard displays agent name with status, message preview (truncated
to 80 chars), relative timestamp, and response indicator (filled/empty
circle). Uses shadcn Card base with selected state highlighting.

packages/web/src/components/MessageCard.tsx

@@ -0,0 +1,83 @@
+import { Card } from "@/components/ui/card";
+import { cn } from "@/lib/utils";
+
+function formatRelativeTime(isoDate: string): string {
+  const now = Date.now();
+  const then = new Date(isoDate).getTime();
+  const diffMs = now - then;
+  const diffSec = Math.floor(diffMs / 1000);
+  const diffMin = Math.floor(diffSec / 60);
+  const diffHr = Math.floor(diffMin / 60);
+  const diffDay = Math.floor(diffHr / 24);
+
+  if (diffSec < 60) return "just now";
+  if (diffMin < 60) return `${diffMin} min ago`;
+  if (diffHr < 24) return `${diffHr}h ago`;
+  return `${diffDay}d ago`;
+}
+
+interface MessageCardProps {
+  agentName: string;
+  agentStatus: string;
+  preview: string;
+  timestamp: string;
+  requiresResponse: boolean;
+  isSelected: boolean;
+  onClick: () => void;
+}
+
+function formatStatusLabel(status: string): string {
+  return status.replace(/_/g, " ");
+}
+
+function truncatePreview(text: string, maxLength = 80): string {
+  if (text.length <= maxLength) return text;
+  return text.slice(0, maxLength) + "...";
+}
+
+export function MessageCard({
+  agentName,
+  agentStatus,
+  preview,
+  timestamp,
+  requiresResponse,
+  isSelected,
+  onClick,
+}: MessageCardProps) {
+  return (
+    <Card
+      className={cn(
+        "cursor-pointer p-4 transition-colors hover:bg-accent/50",
+        isSelected && "bg-accent",
+      )}
+      onClick={onClick}
+    >
+      <div className="flex items-start justify-between gap-4">
+        <div className="min-w-0 flex-1">
+          <div className="flex items-center gap-2">
+            <span
+              className={cn(
+                "text-sm",
+                requiresResponse ? "text-orange-500" : "text-muted-foreground",
+              )}
+            >
+              {requiresResponse ? "\u25CF" : "\u25CB"}
+            </span>
+            <span className="text-sm font-bold">
+              {agentName}{" "}
+              <span className="font-normal text-muted-foreground">
+                ({formatStatusLabel(agentStatus)})
+              </span>
+            </span>
+          </div>
+          <p className="mt-1 pl-5 text-sm text-muted-foreground">
+            &ldquo;{truncatePreview(preview)}&rdquo;
+          </p>
+        </div>
+        <span className="shrink-0 text-xs text-muted-foreground">
+          {formatRelativeTime(timestamp)}
+        </span>
+      </div>
+    </Card>
+  );
+}