feat(worker): add AbortMergeAsync to cancel a conflicted merge

src/ClaudeDo.Worker/Services/TaskMergeService.cs

@@ -22,6 +22,7 @@ public sealed class TaskMergeService
     public const string StatusMerged   = "merged";
     public const string StatusConflict = "conflict";
     public const string StatusBlocked  = "blocked";
+    public const string StatusAborted  = "aborted";
 
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
     private readonly GitService _git;
@@ -195,6 +196,32 @@ public sealed class TaskMergeService
         return new MergeResult(StatusMerged, Array.Empty<string>(), null);
     }
 
+    public async Task<MergeResult> AbortMergeAsync(string taskId, CancellationToken ct)
+    {
+        ListEntity list;
+        WorktreeEntity? wt;
+
+        using (var ctx = _dbFactory.CreateDbContext())
+        {
+            var task = await new TaskRepository(ctx).GetByIdAsync(taskId, ct)
+                ?? throw new KeyNotFoundException($"Task '{taskId}' not found.");
+            list = await new ListRepository(ctx).GetByIdAsync(task.ListId, ct)
+                ?? throw new InvalidOperationException("List not found.");
+            wt = await new WorktreeRepository(ctx).GetByTaskIdAsync(taskId, ct);
+        }
+
+        if (wt is null) return Blocked("task has no worktree");
+        if (wt.State != WorktreeState.Active) return Blocked($"worktree state is {wt.State}");
+        if (string.IsNullOrWhiteSpace(list.WorkingDir)) return Blocked("list has no working directory");
+        if (!await _git.IsMidMergeAsync(list.WorkingDir, ct))
+            return Blocked("repo is not mid-merge");
+
+        await _git.MergeAbortAsync(list.WorkingDir, ct);
+        _logger.LogInformation("Aborted merge of task {TaskId}", taskId);
+
+        return new MergeResult(StatusAborted, Array.Empty<string>(), null);
+    }
+
     public async Task<MergeTargets> GetTargetsAsync(string taskId, CancellationToken ct)
     {
         TaskEntity task;