Remove unused ZMQ command receiver (port 6131)

Service control migrated to SSH, command receiver no longer needed.
- Remove command_receiver Socket from ZmqHandler
- Remove try_receive_command method
- Remove AgentCommand enum
- Remove command_port from ZmqConfig

agent/src/config/mod.rs

@@ -20,7 +20,6 @@ pub struct AgentConfig {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ZmqConfig {
     pub publisher_port: u16,
-    pub command_port: u16,
     pub bind_address: String,
     pub transmission_interval_seconds: u64,
     /// Heartbeat transmission interval in seconds for host connectivity detection