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.Data/Git/GitService.cs

@@ -252,8 +252,11 @@ public sealed class GitService
     public async Task<(int ExitCode, string Stderr)> MergeNoFfAsync(
         string repoDir, string sourceBranch, string message, CancellationToken ct = default)
     {
+        // diff3 conflict style writes the merge base (|||||||) into conflict markers so the
+        // in-app resolver can show a true three-way view. It only enriches conflicted hunks;
+        // clean merges are unaffected.
         var (exitCode, _, stderr) = await RunGitAsync(repoDir,
-            ["merge", "--no-ff", "-m", message, sourceBranch], ct);
+            ["-c", "merge.conflictStyle=diff3", "merge", "--no-ff", "-m", message, sourceBranch], ct);
         return (exitCode, stderr);
     }