fix(merge): refuse a merge that would overwrite an untracked file in the target

Before merging (or staging a conflict resolution's git add -A), compare the
branch's newly-added paths against what's currently untracked in the target
working directory. A collision aborts with a new untracked_collision status
naming the path and size, surfaced through merge_task/review_task,
preview_merge/preview_merge_set (which merge-tree alone can't see), and the
UI merge paths via FlashFooterError/ShowErrorAsync instead of a silent
catch{}. git's own preflight already refuses this while the path stays
untracked at merge time; this closes the gap once a path becomes trackable
in between (e.g. an unrelated conflict resolution's git add -A) or across
the continue_merge TOCTOU window.

src/ClaudeDo.Ui/ViewModels/Conflicts/ConflictResolverViewModel.cs

@@ -143,7 +143,9 @@ public sealed partial class ConflictResolverViewModel : ObservableObject
             var start = await _worker.StartConflictMergeAsync(_taskId, targetBranch);
             if (!string.Equals(start.Status, "conflict", StringComparison.Ordinal))
             {
-                if (string.Equals(start.Status, "blocked", StringComparison.Ordinal))
+                // Any non-conflict outcome here means nothing was left mid-merge for this dialog
+                // to resolve -- surface why instead of quietly closing with no explanation.
+                if (start.Status is "blocked" or "untracked_collision")
                     Error = start.ErrorMessage;
                 return false;
             }