fix(data): use UTF-8 encoding for git process stdio

Ensures non-ASCII git output (branch names, paths, commit messages) is
read and written without locale-dependent corruption.

src/ClaudeDo.Data/Git/GitService.cs

@@ -236,6 +236,9 @@ public sealed class GitService
             RedirectStandardInput = stdinData is not null,
             UseShellExecute = false,
             CreateNoWindow = true,
+            StandardOutputEncoding = Encoding.UTF8,
+            StandardErrorEncoding = Encoding.UTF8,
+            StandardInputEncoding = stdinData is not null ? Encoding.UTF8 : null,
         };
         psi.ArgumentList.Add("-C");
         psi.ArgumentList.Add(workDir);