feat: sanitized agent config — Nexus no longer reads secrets from openclaw.json

- Mount agents-sanitized.json (agents key only, no secrets) instead of full openclaw.json
- Update AgentService default path from /home/node/.openclaw/openclaw.json to /etc/nexus/agents-sanitized.json
- Add AgentConfigPath env var to compose for explicit path configuration
- Generate sanitized file in deploy-nexus.sh before each deploy using Python extraction
- Add agents-sanitized.json to .gitignore

Eliminates the fragile ACL on openclaw.json (uid 1654) that causes 500 errors
on the Board endpoint when lost.

backend/Services/AgentService.cs

@@ -235,7 +235,7 @@ public sealed class AgentService(IConfiguration configuration, IAgentRuntime run
     private async Task<IReadOnlyList<AgentConfig>> LoadAgentConfigsAsync(CancellationToken cancellationToken)
     {
         var path = configuration.GetValue<string>("AgentConfigPath")
-            ?? "/home/node/.openclaw/openclaw.json";
+            ?? "/etc/nexus/agents-sanitized.json";
 
         if (!File.Exists(path))
             return BuildFallbackConfigs();