feat(git): add GetCurrentBranchAsync

src/ClaudeDo.Data/Git/GitService.cs

@@ -151,6 +151,15 @@ public sealed class GitService
             throw new InvalidOperationException($"git branch {flag} failed (exit {exitCode}): {stderr}");
     }
 
+    public async Task<string> GetCurrentBranchAsync(string repoDir, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, stderr) = await RunGitAsync(repoDir,
+            ["symbolic-ref", "--short", "HEAD"], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git symbolic-ref --short HEAD failed (exit {exitCode}): {stderr}");
+        return stdout.Trim();
+    }
+
     public async Task MergeFfOnlyAsync(string repoDir, string branchName, CancellationToken ct = default)
     {
         var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["merge", "--ff-only", branchName], ct);