refactor(agent): move agentdesktop under x/ subpackage (#23610)

- Move `agent/agentdesktop/` to `agent/x/agentdesktop/` to signal
experimental/unstable status
- Update import paths in `agent/agent.go` and `api_test.go`

> 🤖 This mechanical refactor was performed by an agent. I made sure it
didn't change anything it wasn't supposed to.

agent/agent.go

@@ -38,7 +38,6 @@ import (
 	"cdr.dev/slog/v3"
 	"github.com/coder/clistat"
 	"github.com/coder/coder/v2/agent/agentcontainers"
-	"github.com/coder/coder/v2/agent/agentdesktop"
 	"github.com/coder/coder/v2/agent/agentexec"
 	"github.com/coder/coder/v2/agent/agentfiles"
 	"github.com/coder/coder/v2/agent/agentgit"
@@ -50,6 +49,7 @@ import (
 	"github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/agent/proto/resourcesmonitor"
 	"github.com/coder/coder/v2/agent/reconnectingpty"
+	"github.com/coder/coder/v2/agent/x/agentdesktop"
 	"github.com/coder/coder/v2/buildinfo"
 	"github.com/coder/coder/v2/cli/gitauth"
 	"github.com/coder/coder/v2/coderd/database/dbtime"

agent/x/agentdesktop/api_test.go

@@ -15,7 +15,7 @@ import (
 	"golang.org/x/xerrors"
 
 	"cdr.dev/slog/v3/sloggers/slogtest"
-	"github.com/coder/coder/v2/agent/agentdesktop"
+	"github.com/coder/coder/v2/agent/x/agentdesktop"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/codersdk/workspacesdk"
 	"github.com/coder/quartz"