Fix ZMQ socket file descriptor leak in agent heartbeat and IPC

[Copilot]

Agent fails after 3-4 minutes with EMFILE: No file descriptors available (os error 24). Root cause: ReqSocket instances created for each API call (heartbeats every 5s, status updates) were never explicitly closed, relying only on Drop trait which doesn't guarantee immediate FD release.

Changes

zmq_helpers.rs: Explicit socket cleanup in request-reply cycle

let result = match req.send(zmq_msg).await {
    // ... handle send/recv ...
};
// Explicitly close socket to release file descriptors
let close_errors = req.close().await;
if !close_errors.is_empty() {
    warn!("Errors closing ZMQ request socket for {}: {:?}", tcp_uri, close_errors);
}
result

log_manager/publisher.rs: Close old socket before reconnect

• Use std::mem::replace to extract old socket before creating new one
• Call .close() explicitly to prevent FD accumulation during reconnects

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