feat(i18n): add CultureResolver for OS-culture mapping

src/ClaudeDo.Localization/CultureResolver.cs

@@ -0,0 +1,16 @@
+namespace ClaudeDo.Localization;
+
+public static class CultureResolver
+{
+    public static string Resolve(string cultureName, IReadOnlyCollection<string> available, string fallback = "en")
+    {
+        if (string.IsNullOrWhiteSpace(cultureName)) return fallback;
+
+        var exact = available.FirstOrDefault(c => string.Equals(c, cultureName, StringComparison.OrdinalIgnoreCase));
+        if (exact is not null) return exact;
+
+        var primary = cultureName.Split('-')[0];
+        var byPrimary = available.FirstOrDefault(c => string.Equals(c, primary, StringComparison.OrdinalIgnoreCase));
+        return byPrimary ?? fallback;
+    }
+}

tests/ClaudeDo.Localization.Tests/CultureResolverTests.cs

@@ -0,0 +1,19 @@
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Localization.Tests;
+
+public class CultureResolverTests
+{
+    private static readonly string[] Codes = { "en", "de" };
+
+    [Theory]
+    [InlineData("de-DE", "de")]
+    [InlineData("de", "de")]
+    [InlineData("en-US", "en")]
+    [InlineData("fr-FR", "en")]
+    [InlineData("", "en")]
+    public void Resolve_maps_culture_to_available_code(string culture, string expected)
+    {
+        Assert.Equal(expected, CultureResolver.Resolve(culture, Codes, fallback: "en"));
+    }
+}