plan/7: Add active-agent count badge to AgentStatus panel

Adds a count badge next to the Agents section title showing the number
of agents currently in idle or working state (excludes offline).
Badge style matches existing PlanBoard and DirectorQueue badges:
font-mono text-[10px] uppercase tracking-wider text-gray-400.

Build verified clean (tsc + vite, 0 errors).

ui/src/components/AgentStatus.tsx

@@ -18,6 +18,9 @@ export function AgentStatus({ agents }: { agents: Agent[] }) {
     <div className="rounded-lg border border-gray-200 dark:border-gray-700 bg-white dark:bg-gray-800 p-4">
       <div className="flex items-center gap-2 mb-3">
     <h2 className="text-lg font-semibold text-gray-900 dark:text-gray-100">Agents</h2>
+    <span className="font-mono text-[10px] uppercase tracking-wider text-gray-400">
+      {agents.filter(a => a.status === 'idle' || a.status === 'working').length} active
+    </span>
   </div>
       <div className="space-y-2">
         {agents.map((agent) => (