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

tests/ClaudeDo.Ui.Tests/StubWorkerClient.cs

@@ -66,6 +66,7 @@ public abstract class StubWorkerClient : IWorkerClient
     public virtual Task CancelReviewAsync(string taskId) => Task.CompletedTask;
     public virtual Task<MergeResultDto> StartConflictMergeAsync(string taskId, string targetBranch) => Task.FromResult(new MergeResultDto("conflict", System.Array.Empty<string>(), null));
     public virtual Task<MergeConflictsDto> GetMergeConflictsAsync(string taskId) => Task.FromResult(new MergeConflictsDto(taskId, System.Array.Empty<ConflictFileDto>()));
+    public virtual Task<MergeConflictDocumentsDto> GetMergeConflictDocumentsAsync(string taskId) => Task.FromResult(new MergeConflictDocumentsDto(taskId, System.Array.Empty<ConflictDocumentDto>()));
     public virtual Task WriteConflictResolutionAsync(string taskId, string path, string resolvedContent) => Task.CompletedTask;
     public virtual Task<MergeResultDto> ContinueConflictMergeAsync(string taskId) => Task.FromResult(new MergeResultDto("merged", System.Array.Empty<string>(), null));
     public virtual Task AbortConflictMergeAsync(string taskId) => Task.CompletedTask;