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.Ui/Services/Interfaces/IWorkerClient.cs

@@ -55,6 +55,7 @@ public interface IWorkerClient : INotifyPropertyChanged
     // ── Conflict resolution (worker hub side implemented by Layer C) ──
     Task<MergeResultDto> StartConflictMergeAsync(string taskId, string targetBranch);
     Task<MergeConflictsDto> GetMergeConflictsAsync(string taskId);
+    Task<MergeConflictDocumentsDto> GetMergeConflictDocumentsAsync(string taskId);
     Task WriteConflictResolutionAsync(string taskId, string path, string resolvedContent);
     Task<MergeResultDto> ContinueConflictMergeAsync(string taskId);
     Task AbortConflictMergeAsync(string taskId);