refactor: unify agent communication with Transport interface

- Introduce `Transport` interface to abstract WebSocket and SSH
communication
- Add generic `Data` field to `AgentResponse` for streamlined future
endpoints
- Maintain backward compatibility with legacy hubs and agents using
typed fields
- Unify fetch operations (SMART, systemd, containers) under a single
`request` method
- Improve `RequestManager` with deadline awareness and legacy response
support
- Refactor agent response routing into dedicated `agent/response.go`
- Update version to 0.18.0-beta.2

internal/hub/ws/ws.go

@@ -1,6 +1,7 @@
 package ws
 
 import (
+	"context"
 	"errors"
 	"time"
 	"weak"
@@ -161,3 +162,14 @@ func (ws *WsConn) handleAgentRequest(req *PendingRequest, handler ResponseHandle
 func (ws *WsConn) IsConnected() bool {
 	return ws.conn != nil
 }
+
+// AgentVersion returns the connected agent's version (as reported during handshake).
+func (ws *WsConn) AgentVersion() semver.Version {
+	return ws.agentVersion
+}
+
+// SendRequest sends a request to the agent and returns a pending request handle.
+// This is used by the transport layer to send requests.
+func (ws *WsConn) SendRequest(ctx context.Context, action common.WebSocketAction, data any) (*PendingRequest, error) {
+	return ws.requestManager.SendRequest(ctx, action, data)
+}