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

act/container/macos_vm_cleanup.go

@@ -0,0 +1,73 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package container
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os/exec"
+	"strings"
+	"time"
+)
+
+type macOSVM struct {
+	Name       string `json:"name"`
+	State      string `json:"state"`
+	Running    bool   `json:"running"`
+	AccessedAt string `json:"accessed_at"`
+}
+
+// CleanupOrphanMacOSVMs deletes local macOS VMs left behind by this runner that
+// have not been accessed since cutoff.
+func CleanupOrphanMacOSVMs(ctx context.Context, executorPath, uuid string, cutoff time.Time) error {
+	if uuid == "" {
+		return nil
+	}
+
+	output, err := exec.CommandContext(ctx, executorPath, "list", "--format", "json").Output()
+	if err != nil {
+		return fmt.Errorf("list macOS VMs: %w", err)
+	}
+
+	var vms []macOSVM
+	if err := json.Unmarshal(output, &vms); err != nil {
+		return fmt.Errorf("parse macOS VM list: %w", err)
+	}
+
+	var errs []error
+	for _, name := range macOSVMsToDelete(vms, uuid, cutoff) {
+		if err := exec.CommandContext(ctx, executorPath, "destroy", name).Run(); err != nil {
+			errs = append(errs, fmt.Errorf("destroy macOS VM %s: %w", name, err))
+		}
+	}
+	return errors.Join(errs...)
+}
+
+func macOSVMsToDelete(vms []macOSVM, uuid string, cutoff time.Time) []string {
+	prefix := "GITEA-MACOS-VM-" + uuid + "-"
+	var stale []string
+	for _, vm := range vms {
+		if vm.Running || !strings.HasPrefix(vm.Name, prefix) {
+			continue
+		}
+		accessed, err := parseMacOSVMTime(vm.AccessedAt)
+		if err != nil || accessed.After(cutoff) {
+			continue
+		}
+		stale = append(stale, vm.Name)
+	}
+	return stale
+}
+
+func parseMacOSVMTime(raw string) (time.Time, error) {
+	if raw == "" {
+		return time.Time{}, errors.New("empty access time")
+	}
+	if parsed, err := time.Parse(time.RFC3339Nano, raw); err == nil {
+		return parsed, nil
+	}
+	return time.Parse(time.RFC3339, raw)
+}