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/Hub/WorkerHub.cs

@@ -61,6 +61,9 @@ public record MergeTargetsDto(string DefaultBranch, IReadOnlyList<string> LocalB
 public record MergeConflictsDto(string TaskId, IReadOnlyList<ConflictFileDto> Files);
 public record ConflictFileDto(string Path, IReadOnlyList<ConflictHunkDto> Hunks);
 public record ConflictHunkDto(string Ours, string Theirs, string? Base);
+public record MergeConflictDocumentsDto(string TaskId, IReadOnlyList<ConflictDocumentDto> Files);
+public record ConflictDocumentDto(string Path, bool IsBinary, IReadOnlyList<MergeSegmentDto> Segments);
+public record MergeSegmentDto(bool IsConflict, string Text, string Ours, string? Base, string Theirs);
 public record UpdateListDto(string Id, string Name, string? WorkingDir, string DefaultCommitType);
 public record UpdateListConfigDto(string ListId, string? Model, string? SystemPrompt, string? AgentPath, int? MaxTurns = null);
 public record UpdateTaskAgentSettingsDto(string TaskId, string? Model, string? SystemPrompt, string? AgentPath, int? MaxTurns = null);
@@ -383,6 +386,18 @@ public sealed class WorkerHub : Microsoft.AspNetCore.SignalR.Hub
                     new[] { new ConflictHunkDto(f.Ours, f.Theirs, f.Base) })).ToList());
         });
 
+    public Task<MergeConflictDocumentsDto> GetMergeConflictDocuments(string taskId)
+        => HubGuard(async () =>
+        {
+            var c = await _mergeService.GetConflictDocumentsAsync(taskId, CancellationToken.None);
+            return new MergeConflictDocumentsDto(
+                c.TaskId,
+                c.Files.Select(f => new ConflictDocumentDto(
+                    f.Path, f.IsBinary,
+                    f.Segments.Select(s => new MergeSegmentDto(
+                        s.IsConflict, s.Text, s.Ours, s.Base, s.Theirs)).ToList())).ToList());
+        });
+
     public Task WriteConflictResolution(string taskId, string path, string resolvedContent)
         => HubGuard(() => _mergeService.WriteResolutionAsync(
             taskId, path, resolvedContent ?? "", CancellationToken.None));