feat(git): add non-destructive merge-tree conflict probe

src/ClaudeDo.Data/Git/GitService.cs

@@ -3,6 +3,8 @@ using System.Text;
 
 namespace ClaudeDo.Data.Git;
 
+public sealed record MergePreview(bool Supported, bool Clean, IReadOnlyList<string> ConflictFiles);
+
 public sealed class GitService
 {
     public async Task<bool> IsGitRepoAsync(string dir, CancellationToken ct = default)
@@ -236,6 +238,49 @@ public sealed class GitService
             .ToList();
     }
 
+    /// <summary>
+    /// Non-destructive mergeability probe via `git merge-tree --write-tree`. Writes only
+    /// loose objects — the working tree, index, and refs are left untouched.
+    /// </summary>
+    public async Task<MergePreview> PreviewMergeAsync(
+        string repoDir, string targetBranch, string sourceBranch, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir,
+            ["merge-tree", "--write-tree", "--name-only", targetBranch, sourceBranch], ct);
+
+        if (exitCode == 0)
+            return new MergePreview(true, true, Array.Empty<string>());
+
+        if (exitCode == 1)
+        {
+            // stdout: <tree-oid>\n<file>\n...\n\n<informational messages>
+            var lines = stdout.Split('\n');
+            var files = new List<string>();
+            for (int i = 1; i < lines.Length; i++)
+            {
+                var line = lines[i].TrimEnd('\r');
+                if (string.IsNullOrWhiteSpace(line)) break;
+                files.Add(line.Trim());
+            }
+            return new MergePreview(true, false, files);
+        }
+
+        // Any other exit (e.g. git too old: "unknown option --write-tree").
+        return new MergePreview(false, false, Array.Empty<string>());
+    }
+
+    /// <summary>Count of files that differ on <paramref name="sourceBranch"/> since its merge base with the target.</summary>
+    public async Task<int> CountChangedFilesAsync(
+        string repoDir, string targetBranch, string sourceBranch, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir,
+            ["diff", "--name-only", $"{targetBranch}...{sourceBranch}"], ct);
+        if (exitCode != 0) return 0;
+        return stdout
+            .Split('\n', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Count(s => s.Length > 0);
+    }
+
     public async Task MergeFfOnlyAsync(string repoDir, string branchName, CancellationToken ct = default)
     {
         var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["merge", "--ff-only", branchName], ct);

tests/ClaudeDo.Worker.Tests/Runner/GitServicePreviewMergeTests.cs

@@ -0,0 +1,64 @@
+using ClaudeDo.Data.Git;
+using ClaudeDo.Worker.Tests.Infrastructure;
+
+namespace ClaudeDo.Worker.Tests.Runner;
+
+public class GitServicePreviewMergeTests : IDisposable
+{
+    private readonly List<GitRepoFixture> _repos = new();
+    private GitRepoFixture NewRepo() { var r = new GitRepoFixture(); _repos.Add(r); return r; }
+    public void Dispose() { foreach (var r in _repos) try { r.Dispose(); } catch { } }
+
+    [Fact]
+    public async Task PreviewMergeAsync_NonConflicting_ReportsCleanWithChangedCount()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) return;
+        var repo = NewRepo();
+        var git = new GitService();
+        var baseBranch = await git.GetCurrentBranchAsync(repo.RepoDir);
+
+        GitRepoFixture.RunGit(repo.RepoDir, "checkout", "-b", "feature");
+        File.WriteAllText(Path.Combine(repo.RepoDir, "newfile.txt"), "x\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "-A");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "feat");
+        GitRepoFixture.RunGit(repo.RepoDir, "checkout", baseBranch);
+
+        var preview = await git.PreviewMergeAsync(repo.RepoDir, baseBranch, "feature", CancellationToken.None);
+
+        Assert.True(preview.Supported);
+        Assert.True(preview.Clean);
+        Assert.Empty(preview.ConflictFiles);
+
+        var count = await git.CountChangedFilesAsync(repo.RepoDir, baseBranch, "feature", CancellationToken.None);
+        Assert.Equal(1, count);
+    }
+
+    [Fact]
+    public async Task PreviewMergeAsync_Conflicting_ReportsFilesAndDoesNotMutateTree()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) return;
+        var repo = NewRepo();
+        var git = new GitService();
+        var baseBranch = await git.GetCurrentBranchAsync(repo.RepoDir);
+
+        GitRepoFixture.RunGit(repo.RepoDir, "checkout", "-b", "feature");
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "# from feature\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "-A");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "feat readme");
+        GitRepoFixture.RunGit(repo.RepoDir, "checkout", baseBranch);
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "# from base\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "-A");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "base readme");
+
+        var headBefore = GitRepoFixture.RunGit(repo.RepoDir, "rev-parse", "HEAD").Trim();
+
+        var preview = await git.PreviewMergeAsync(repo.RepoDir, baseBranch, "feature", CancellationToken.None);
+
+        Assert.True(preview.Supported);
+        Assert.False(preview.Clean);
+        Assert.Contains("README.md", preview.ConflictFiles);
+
+        Assert.Equal(headBefore, GitRepoFixture.RunGit(repo.RepoDir, "rev-parse", "HEAD").Trim());
+        Assert.False(await git.IsMidMergeAsync(repo.RepoDir));
+    }
+}