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/cluster.go

@@ -0,0 +1,22 @@
+package preflight
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// LooksClustered does a conservative, heuristic scan for cluster-related
+// configuration. It exists to force a manual confirmation gate (see
+// ARCHITECTURE.md §4.1's cluster gate: one live node on the old version
+// during migration corrupts a shared store), not to enumerate peers
+// precisely. A false positive just costs an extra confirmation prompt; a
+// false negative is the dangerous direction, so this errs toward matching
+// broadly rather than requiring an exact schema match.
+func LooksClustered(configPath string) (bool, error) {
+	data, err := os.ReadFile(configPath)
+	if err != nil {
+		return false, fmt.Errorf("preflight: read config %s: %w", configPath, err)
+	}
+	return strings.Contains(strings.ToLower(string(data)), "cluster"), nil
+}