feat(cli): scadalink audit query subcommand (#23 M8)

src/ScadaLink.CLI/Commands/AuditVerifyChainHelpers.cs

@@ -0,0 +1,20 @@
+using System.Globalization;
+
+namespace ScadaLink.CLI.Commands;
+
+/// <summary>
+/// Helpers for the <c>audit verify-chain</c> subcommand. v1 is a no-op: hash-chain
+/// tamper-evidence is deferred to v1.x (see Component-AuditLog.md). The command still
+/// validates its <c>--month</c> argument so the surface is stable for v1.x.
+/// </summary>
+public static class AuditVerifyChainHelpers
+{
+    /// <summary>
+    /// Returns true if <paramref name="month"/> is a well-formed <c>YYYY-MM</c> value
+    /// with a real month (01-12). A malformed month (e.g. <c>2026-13</c>) is rejected.
+    /// </summary>
+    public static bool IsValidMonth(string? month)
+        => !string.IsNullOrWhiteSpace(month)
+           && DateTime.TryParseExact(month, "yyyy-MM", CultureInfo.InvariantCulture,
+               DateTimeStyles.None, out _);
+}