feat: synchronous replay endpoint with agent response status

Add dedicated POST /agents/{id}/replay endpoint that uses
addCommandWithReply to wait for the agent ACK (30s timeout).
Returns the actual replay result (status, message, data) instead
of just a delivery confirmation.

Frontend toast now reflects the agent's response: "Replay completed"
on success, agent error message on failure, timeout message if the
agent doesn't respond.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Exchanges/RouteControlBar.tsx

@@ -57,8 +57,12 @@ export function RouteControlBar({ application, routeId, hasRouteControl, hasRepl
     replayExchange.mutate(
       { agentId, routeId, headers, body: inputBody ?? '', originalExchangeId: exchangeId },
       {
-        onSuccess: () => {
-          toast({ title: 'Replay sent', description: `${routeId} on ${agentId}`, variant: 'success' });
+        onSuccess: (result) => {
+          if (result.status === 'SUCCESS') {
+            toast({ title: 'Replay completed', description: result.message ?? `${routeId} on ${agentId}`, variant: 'success' });
+          } else {
+            toast({ title: 'Replay failed', description: result.message ?? 'Agent reported failure', variant: 'error' });
+          }
           setSendingAction(null);
         },
         onError: (err) => {