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

agent/response.go

@@ -0,0 +1,31 @@
+package agent
+
+import (
+	"github.com/fxamacker/cbor/v2"
+	"github.com/henrygd/beszel/internal/common"
+	"github.com/henrygd/beszel/internal/entities/smart"
+	"github.com/henrygd/beszel/internal/entities/system"
+	"github.com/henrygd/beszel/internal/entities/systemd"
+)
+
+// newAgentResponse creates an AgentResponse using legacy typed fields.
+// This maintains backward compatibility with <= 0.17 hubs that expect specific fields.
+func newAgentResponse(data any, requestID *uint32) common.AgentResponse {
+	response := common.AgentResponse{Id: requestID}
+	switch v := data.(type) {
+	case *system.CombinedData:
+		response.SystemData = v
+	case *common.FingerprintResponse:
+		response.Fingerprint = v
+	case string:
+		response.String = &v
+	case map[string]smart.SmartData:
+		response.SmartData = v
+	case systemd.ServiceDetails:
+		response.ServiceInfo = v
+	default:
+		// For unknown types, use the generic Data field
+		response.Data, _ = cbor.Marshal(data)
+	}
+	return response
+}