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/server.go

@@ -13,9 +13,7 @@ import (
 
 	"github.com/henrygd/beszel"
 	"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"
 
 	"github.com/blang/semver"
 	"github.com/fxamacker/cbor/v2"
@@ -165,20 +163,9 @@ func (a *Agent) handleSSHRequest(w io.Writer, req *common.HubRequest[cbor.RawMes
 	}
 
 	// responder that writes AgentResponse to stdout
+	// Uses legacy typed fields for backward compatibility with <= 0.17
 	sshResponder := func(data any, requestID *uint32) error {
-		response := common.AgentResponse{Id: requestID}
-		switch v := data.(type) {
-		case *system.CombinedData:
-			response.SystemData = v
-		case string:
-			response.String = &v
-		case map[string]smart.SmartData:
-			response.SmartData = v
-		case systemd.ServiceDetails:
-			response.ServiceInfo = v
-		default:
-			response.Error = fmt.Sprintf("unsupported response type: %T", data)
-		}
+		response := newAgentResponse(data, requestID)
 		return cbor.NewEncoder(w).Encode(response)
 	}