feat(mcp): expose the conflict resolver's hunk API over MCP

get_merge_conflicts and resolve_conflict_hunk let an MCP caller resolve a
leaveConflictsInTree merge hunk-by-hunk (ours/theirs/base/literal text)
instead of grep-parsing diff3 markers itself. Reuses the existing
TaskMergeService/ConflictMarkerParser machinery (adds MergeSegment.StartLine
and a non-staging TaskMergeService.WriteConflictFileAsync) rather than a
second parser. A partially resolved file is written without git add so
continue_merge keeps refusing while any hunk still has markers.

src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

@@ -488,6 +488,24 @@ public sealed class TaskMergeService
         await _git.AddPathAsync(list.WorkingDir, path, ct);
     }
 
+    /// <summary>
+    /// Writes a conflicted file's content without staging it. Unlike <see cref="WriteResolutionAsync"/> this must
+    /// NOT run `git add` — that marks the path resolved in git's index regardless of its content, so a partially
+    /// resolved hunk (markers still present elsewhere in the file) would drop out of
+    /// <see cref="GetConflictDocumentsAsync"/>'s conflicted-file list and <see cref="ContinueMergeAsync"/>'s
+    /// pre-stage marker scan, letting a still-conflicted file slip into a commit. Staging happens once, for
+    /// everything, inside <see cref="ContinueMergeAsync"/>.
+    /// </summary>
+    public async Task WriteConflictFileAsync(string taskId, string path, string content, CancellationToken ct)
+    {
+        var (_, list, _, _) = await LoadMergeContextAsync(taskId, ct);
+        if (string.IsNullOrWhiteSpace(list.WorkingDir))
+            throw new InvalidOperationException("list has no working directory");
+
+        var full = Path.Combine(list.WorkingDir, path.Replace('/', Path.DirectorySeparatorChar));
+        await File.WriteAllTextAsync(full, content, ct);
+    }
+
     public async Task<MergeTargets> GetTargetsAsync(string taskId, CancellationToken ct)
     {
         var (_, list, _, _) = await LoadMergeContextAsync(taskId, ct);