fix(agents): finish sanitized config migration — GatewayClient also reads agents-sanitized.json

The previous commit f4bee44 updated AgentService.cs but missed
OpenClawGatewayClient.cs which also reads agent IDs and model config
from the raw openclaw.json. Both LoadAgentIdsFromConfig() and
GetAvailableModels() now default to /etc/nexus/agents-sanitized.json.

This completes the P4 migration: Nexus no longer needs read access to
openclaw.json for any code path.

backend/Services/OpenClawGatewayClient.cs

@@ -123,7 +123,7 @@ public sealed class OpenClawGatewayClient(HttpClient httpClient, IConfiguration
             )
         };
 
-        // Load agent IDs from openclaw.json config
+        // Load agent IDs from sanitized agents config (no secrets)
         var agentIds = LoadAgentIdsFromConfig();
 
         var agents = new List<DashboardAgentInfo>();
@@ -227,7 +227,7 @@ public sealed class OpenClawGatewayClient(HttpClient httpClient, IConfiguration
     }
 
     /// <summary>
-    /// Loads agent IDs from the OpenClaw config file (openclaw.json).
+    /// Loads agent IDs from the sanitized agents config (no secrets).
     /// Falls back to the known list if the config file is unavailable.
     /// </summary>
     private List<string> LoadAgentIdsFromConfig()
@@ -235,7 +235,7 @@ public sealed class OpenClawGatewayClient(HttpClient httpClient, IConfiguration
         try
         {
             var configPath = configuration.GetValue<string>("AgentConfigPath")
-                ?? "/home/node/.openclaw/openclaw.json";
+                ?? "/etc/nexus/agents-sanitized.json";
 
             if (!System.IO.File.Exists(configPath))
                 return GetDefaultAgentIds();
@@ -1085,7 +1085,7 @@ public sealed class OpenClawGatewayClient(HttpClient httpClient, IConfiguration
         try
         {
             var configPath = configuration.GetValue<string>("AgentConfigPath")
-                ?? "/home/node/.openclaw/openclaw.json";
+                ?? "/etc/nexus/agents-sanitized.json";
 
             if (!System.IO.File.Exists(configPath))
                 return GetDefaultModels();