fix(worker): return Blocked when MergeAbortAsync fails to avoid stuck repo

If git merge --abort throws, the repo is left mid-merge. Previously the
code logged a warning and returned a conflict result, giving the UI a
stale file list. Now it returns Blocked with an explicit message so the
caller knows manual resolution is required.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Worker/Services/TaskMergeService.cs

@@ -88,8 +88,15 @@ public sealed class TaskMergeService
             List<string> files;
             try { files = await _git.ListConflictedFilesAsync(list.WorkingDir, ct); }
             catch { files = new(); }
+
+            // If abort fails the repo is left mid-merge; the caller must resolve manually.
+            // Return Blocked (not conflict) so the UI does not offer a stale conflict list.
             try { await _git.MergeAbortAsync(list.WorkingDir, ct); }
-            catch (Exception ex) { _logger.LogWarning(ex, "git merge --abort failed after conflict"); }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "git merge --abort failed after conflict — repo is mid-merge");
+                return Blocked($"merge conflict and abort failed: {ex.Message} — repo is mid-merge, resolve manually");
+            }
 
             if (files.Count == 0)
             {