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/api/queries/commands.ts

@@ -175,6 +175,12 @@ export function useSendRouteCommand() {
 
 // ── Replay Exchange ───────────────────────────────────────────────────────
 
+export interface ReplayResult {
+  status: string
+  message: string
+  data?: string
+}
+
 export function useReplayExchange() {
   return useMutation({
     mutationFn: async ({
@@ -189,21 +195,18 @@ export function useReplayExchange() {
       headers?: Record<string, string>
       body: string
       originalExchangeId?: string
-    }) => {
-      const { data, error } = await api.POST('/agents/{id}/commands', {
-        params: { path: { id: agentId } },
-        body: {
-          type: 'replay',
-          payload: {
-            routeId,
-            exchange: { body, headers: headers ?? {} },
-            originalExchangeId,
-            nonce: crypto.randomUUID(),
-          },
-        } as any,
+    }): Promise<ReplayResult> => {
+      const res = await authFetch(`/api/v1/agents/${encodeURIComponent(agentId)}/replay`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ routeId, body, headers: headers ?? {}, originalExchangeId }),
       })
-      if (error) throw new Error('Failed to send replay command')
-      return data!
+      if (!res.ok) {
+        if (res.status === 404) throw new Error('Agent not found')
+        if (res.status === 504) throw new Error('Replay timed out — agent did not respond')
+        throw new Error('Failed to send replay command')
+      }
+      return res.json() as Promise<ReplayResult>
     },
   })
 }