Initial commit: stalwart-migrator design and scaffolding

In-place upgrade tool for Stalwart Mail Server (0.15.5 -> latest) with
checkpointed rollback and post-migration validation. Design stage; see
ARCHITECTURE.md.

internal/preflight/deployment.go

@@ -0,0 +1,45 @@
+package preflight
+
+import (
+	"context"
+	"os"
+	"os/exec"
+)
+
+// DeploymentKind is how a Stalwart instance appears to be run, which
+// determines how cutover and rollback restart it.
+type DeploymentKind string
+
+const (
+	DeploymentSystemd DeploymentKind = "systemd"
+	DeploymentDocker  DeploymentKind = "docker"
+	DeploymentUnknown DeploymentKind = "unknown"
+)
+
+var systemdUnitPaths = []string{
+	"/etc/systemd/system/stalwart.service",
+	"/lib/systemd/system/stalwart.service",
+	"/usr/lib/systemd/system/stalwart.service",
+}
+
+// DetectDeploymentKind makes a best-effort guess at how Stalwart is run
+// here. It's deliberately conservative and cheap (file stats, one docker
+// inspect) rather than exhaustive - an operator-supplied override should
+// always be able to win over this, since the cost of guessing wrong here is
+// cutover targeting the wrong thing.
+func DetectDeploymentKind(ctx context.Context, containerName string) DeploymentKind {
+	for _, p := range systemdUnitPaths {
+		if _, err := os.Stat(p); err == nil {
+			return DeploymentSystemd
+		}
+	}
+	if containerName == "" {
+		containerName = "stalwart"
+	}
+	if _, err := exec.LookPath("docker"); err == nil {
+		if err := exec.CommandContext(ctx, "docker", "inspect", containerName).Run(); err == nil {
+			return DeploymentDocker
+		}
+	}
+	return DeploymentUnknown
+}