feat(merge): real conflict-hunk parsing pipeline (chunk 2 backend)

Replace the whole-file conflict model with line-level hunks, the
foundation for the full in-app merge editor.

- ConflictMarkerParser: parses git conflict markers (incl. diff3 base)
  into ordered stable/conflict MergeSegments; exact round-trip + Compose
- GitService.MergeNoFfAsync passes -c merge.conflictStyle=diff3 so the
  working tree carries the merge base in conflict markers
- TaskMergeService.GetConflictDocumentsAsync: reads each conflicted file,
  parses into segments, flags binary files
- hub GetMergeConflictDocuments + DTOs (MergeConflictDocumentsDto/
  ConflictDocumentDto/MergeSegmentDto), IWorkerClient + both fakes
- tests: 8 parser unit tests + a real-git integration test asserting
  line-level hunks with a diff3 base

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -33,6 +33,15 @@ public sealed record ConflictFileContent(
     string Theirs,
     string? Base);
 
+public sealed record ConflictDocuments(
+    string TaskId,
+    IReadOnlyList<ConflictDocumentContent> Files);
+
+public sealed record ConflictDocumentContent(
+    string Path,
+    bool IsBinary,
+    IReadOnlyList<MergeSegment> Segments);
+
 public sealed class TaskMergeService
 {
     public const string StatusMerged   = "merged";
@@ -256,6 +265,45 @@ public sealed class TaskMergeService
         return new MergeConflicts(taskId, result);
     }
 
+    /// <summary>
+    /// Reads each conflicted working-tree file and parses its conflict markers into line-level
+    /// segments (with the diff3 merge base when present). Binary files are flagged and skipped.
+    /// </summary>
+    public async Task<ConflictDocuments> GetConflictDocumentsAsync(string taskId, CancellationToken ct)
+    {
+        var (_, list, _) = await LoadMergeContextAsync(taskId, ct);
+        if (string.IsNullOrWhiteSpace(list.WorkingDir))
+            throw new InvalidOperationException("list has no working directory");
+
+        var files = await _git.ListConflictedFilesAsync(list.WorkingDir, ct);
+        var result = new List<ConflictDocumentContent>(files.Count);
+        foreach (var path in files)
+        {
+            var full = Path.Combine(list.WorkingDir, path.Replace('/', Path.DirectorySeparatorChar));
+            string text;
+            try { text = await File.ReadAllTextAsync(full, ct); }
+            catch { text = ""; }
+
+            if (LooksBinary(text))
+            {
+                result.Add(new ConflictDocumentContent(path, true, Array.Empty<MergeSegment>()));
+                continue;
+            }
+
+            result.Add(new ConflictDocumentContent(path, false, ConflictMarkerParser.Parse(text)));
+        }
+        return new ConflictDocuments(taskId, result);
+    }
+
+    // A NUL byte in the head of the file is the conventional binary sniff.
+    private static bool LooksBinary(string text)
+    {
+        var n = Math.Min(text.Length, 8000);
+        for (var i = 0; i < n; i++)
+            if (text[i] == '\0') return true;
+        return false;
+    }
+
     public async Task WriteResolutionAsync(string taskId, string path, string content, CancellationToken ct)
     {
         var (_, list, _) = await LoadMergeContextAsync(taskId, ct);