feat(ui): add RepoScanner for git repo discovery

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Services/RepoScanner.cs

@@ -0,0 +1,25 @@
+namespace ClaudeDo.Ui.Services;
+
+public sealed record RepoCandidate(string Name, string FullPath);
+
+public static class RepoScanner
+{
+    public static IReadOnlyList<RepoCandidate> Scan(string parentFolder)
+    {
+        if (string.IsNullOrWhiteSpace(parentFolder) || !Directory.Exists(parentFolder))
+            return Array.Empty<RepoCandidate>();
+
+        var result = new List<RepoCandidate>();
+        IEnumerable<string> subdirs;
+        try { subdirs = Directory.EnumerateDirectories(parentFolder); }
+        catch { return Array.Empty<RepoCandidate>(); }
+
+        foreach (var dir in subdirs)
+        {
+            var gitPath = Path.Combine(dir, ".git");
+            if (Directory.Exists(gitPath) || File.Exists(gitPath))
+                result.Add(new RepoCandidate(Path.GetFileName(dir), dir));
+        }
+        return result;
+    }
+}