enhance: add macOS VM executor backend

Replace the Tart-based prototype with a runner-oriented macOS VM
executor backed by runner-executor-macos and Apple Virtualization.

Assisted-by: Codet:MODEL_VERSION

internal/pkg/labels/labels.go

@@ -9,11 +9,15 @@ import (
 )
 
 const (
-	SchemeHost   = "host"
-	SchemeDocker = "docker"
+	SchemeHost    = "host"
+	SchemeDocker  = "docker"
+	SchemeMacOSVM = "macos-vm"
 
 	// SelfHostedPlatform is the platform marker act treats as "run on the host".
 	SelfHostedPlatform = "-self-hosted"
+
+	// MacOSVMSchemePrefix marks a platform image that act should run in a macOS VM.
+	MacOSVMSchemePrefix = "macos-vm://"
 )
 
 type Label struct {
@@ -42,7 +46,7 @@ func Parse(str string) (*Label, error) {
 	if len(splits) >= 3 {
 		label.Arg = splits[2]
 	}
-	if label.Schema != SchemeHost && label.Schema != SchemeDocker {
+	if label.Schema != SchemeHost && label.Schema != SchemeDocker && label.Schema != SchemeMacOSVM {
 		// Not a schema we know: the colon belongs to the label name itself.
 		return &Label{
 			Name:   str,
@@ -64,6 +68,15 @@ func (l Labels) RequireDocker() bool {
 	return false
 }
 
+func (l Labels) RequireMacOSVM() bool {
+	for _, label := range l {
+		if label.Schema == SchemeMacOSVM {
+			return true
+		}
+	}
+	return false
+}
+
 // PickPlatform returns the platform of the first runs-on entry this runner has a label for, or "".
 func (l Labels) PickPlatform(runsOn []string) string {
 	platforms := make(map[string]string, len(l))
@@ -72,10 +85,12 @@ func (l Labels) PickPlatform(runsOn []string) string {
 		case SchemeDocker:
 			// "//" will be ignored
 			platforms[label.Name] = strings.TrimPrefix(label.Arg, "//")
+		case SchemeMacOSVM:
+			platforms[label.Name] = MacOSVMSchemePrefix + strings.TrimPrefix(label.Arg, "//")
 		case SchemeHost:
 			platforms[label.Name] = SelfHostedPlatform
 		default:
-			// unreachable: Parse only produces host or docker schemas
+			// unreachable: Parse only produces host, docker, or macOS VM schemas
 			continue
 		}
 	}

internal/pkg/labels/labels_test.go

@@ -25,6 +25,15 @@ func TestParse(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			args: "macos-latest:macos-vm://ghcr.io/cirruslabs/macos-sonoma-base:latest",
+			want: &Label{
+				Name:   "macos-latest",
+				Schema: "macos-vm",
+				Arg:    "//ghcr.io/cirruslabs/macos-sonoma-base:latest",
+			},
+			wantErr: false,
+		},
 		{
 			args: "ubuntu:host",
 			want: &Label{
@@ -103,6 +112,7 @@ func TestRequireDocker(t *testing.T) {
 		{"empty", nil, false},
 		{"only host", []string{"ubuntu:host", "self-hosted"}, false},
 		{"has docker", []string{"ubuntu:host", "ubuntu:docker://node:18"}, true},
+		{"macos-vm does not require docker", []string{"macos-latest:macos-vm://ghcr.io/cirruslabs/macos-sonoma-base:latest"}, false},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -111,10 +121,29 @@ func TestRequireDocker(t *testing.T) {
 	}
 }
 
+func TestRequireMacOSVM(t *testing.T) {
+	tests := []struct {
+		name string
+		strs []string
+		want bool
+	}{
+		{"empty", nil, false},
+		{"only host", []string{"ubuntu:host", "self-hosted"}, false},
+		{"only docker", []string{"ubuntu:docker://node:18"}, false},
+		{"has macos-vm", []string{"macos-latest:macos-vm://ghcr.io/cirruslabs/macos-sonoma-base:latest"}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, mustParse(t, tt.strs...).RequireMacOSVM())
+		})
+	}
+}
+
 func TestPickPlatform(t *testing.T) {
 	ls := mustParse(t,
 		"ubuntu:docker://node:18",
 		"self-hosted:host",
+		"macos-latest:macos-vm://ghcr.io/cirruslabs/macos-sonoma-base:latest",
 	)
 
 	tests := []struct {
@@ -124,6 +153,7 @@ func TestPickPlatform(t *testing.T) {
 	}{
 		{"docker strips leading slashes", []string{"ubuntu"}, "node:18"},
 		{"host maps to self-hosted marker", []string{"self-hosted"}, SelfHostedPlatform},
+		{"macos-vm prefixes platform marker", []string{"macos-latest"}, MacOSVMSchemePrefix + "ghcr.io/cirruslabs/macos-sonoma-base:latest"},
 		{"first match wins", []string{"self-hosted", "ubuntu"}, SelfHostedPlatform},
 		{"unknown label picks nothing", []string{"windows"}, ""},
 		{"no runsOn picks nothing", nil, ""},
@@ -145,12 +175,14 @@ func TestToStrings(t *testing.T) {
 	ls := mustParse(t,
 		"ubuntu:docker://node:18",
 		"self-hosted:host",
+		"macos-latest:macos-vm://ghcr.io/cirruslabs/macos-sonoma-base:latest",
 		"bare",
 		"pool:e57e18d4",
 	)
 	require.Equal(t, []string{
 		"ubuntu:docker://node:18",
 		"self-hosted:host",
+		"macos-latest:macos-vm://ghcr.io/cirruslabs/macos-sonoma-base:latest",
 		"bare:host",
 		"pool:e57e18d4",
 	}, ls.ToStrings())