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.Worker.Tests/Services/TaskMergeServiceTests.cs

@@ -571,6 +571,51 @@ public class TaskMergeServiceTests : IDisposable
         Assert.False(await new GitService().IsMidMergeAsync(repo.RepoDir));
     }
 
+    [Fact]
+    public async Task GetConflictDocumentsAsync_ParsesLineLevelHunksWithDiff3Base()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) return;
+        var repo = NewRepo();
+        var db = NewDb();
+        var (list, task) = await SeedListAndTask(db, repo.RepoDir, TaskStatus.WaitingForReview);
+
+        // A committed README gives the conflict a common ancestor, so diff3 records a base.
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "line1\nbase\nline3\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "-A");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "base readme");
+
+        var wtMgr = BuildWorktreeManager(db);
+        var wtCtx = await wtMgr.CreateAsync(task, list, CancellationToken.None);
+        _wtCleanups.Add((repo.RepoDir, wtCtx.WorktreePath));
+        File.WriteAllText(Path.Combine(wtCtx.WorktreePath, "README.md"), "line1\nWORKTREE\nline3\n");
+        await wtMgr.CommitIfChangedAsync(wtCtx, task, list, CancellationToken.None);
+
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "line1\nMAIN\nline3\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "add", "-A");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-m", "main edit");
+
+        var (svc, _) = BuildService(db);
+        var target = await new GitService().GetCurrentBranchAsync(repo.RepoDir);
+
+        var merge = await svc.MergeAsync(
+            task.Id, target, removeWorktree: false, "merge",
+            leaveConflictsInTree: true, CancellationToken.None);
+        Assert.Equal(TaskMergeService.StatusConflict, merge.Status);
+
+        var docs = await svc.GetConflictDocumentsAsync(task.Id, CancellationToken.None);
+
+        var file = Assert.Single(docs.Files);
+        Assert.EndsWith("README.md", file.Path.Replace('\\', '/'));
+        Assert.False(file.IsBinary);
+
+        var conflict = Assert.Single(file.Segments.Where(s => s.IsConflict).ToList());
+        Assert.Contains("MAIN", conflict.Ours);          // ours = current branch (merge target)
+        Assert.Contains("WORKTREE", conflict.Theirs);     // theirs = incoming worktree branch
+        Assert.NotNull(conflict.Base);
+        Assert.Contains("base", conflict.Base!);          // diff3 merge base
+        Assert.Contains(file.Segments, s => !s.IsConflict && s.Text.Contains("line1"));
+    }
+
     [Fact]
     public async Task ApproveAndMergeAsync_NoWorktree_MarksDone()
     {