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;

tests/ClaudeDo.Worker.Tests/Services/TaskMergeServiceTests.cs

@@ -408,6 +408,40 @@ public class TaskMergeServiceTests : IDisposable
         Assert.Equal(WorktreeState.Merged, wt.State);
     }
 
+    [Fact]
+    public async Task AbortMergeAsync_AfterConflict_RestoresCleanStateAndLeavesWorktreeActive()
+    {
+        if (!GitRepoFixture.IsGitAvailable()) return;
+
+        var db = NewDb();
+        var repo = NewRepo();
+        GitRepoFixture.RunGit(repo.RepoDir, "branch", "-m", "main");
+
+        File.WriteAllText(Path.Combine(repo.RepoDir, "README.md"), "# main change\n");
+        GitRepoFixture.RunGit(repo.RepoDir, "commit", "-am", "main change");
+
+        var wtPath = Path.Combine(Path.GetTempPath(), $"wt_{Guid.NewGuid():N}");
+        _wtCleanups.Add((repo.RepoDir, wtPath));
+        GitRepoFixture.RunGit(repo.RepoDir, "worktree", "add", "-b", "claudedo/t3", wtPath, repo.BaseCommit);
+        File.WriteAllText(Path.Combine(wtPath, "README.md"), "# branch change\n");
+        GitRepoFixture.RunGit(wtPath, "commit", "-am", "branch change");
+
+        var (_, task) = await SeedListAndTask(db, workingDir: repo.RepoDir, status: TaskStatus.Done);
+        await SeedWorktree(db, task.Id, wtPath, "claudedo/t3", repo.BaseCommit);
+
+        var (svc, _) = BuildService(db);
+        await svc.MergeAsync(task.Id, "main", false, "msg", leaveConflictsInTree: true, CancellationToken.None);
+
+        var result = await svc.AbortMergeAsync(task.Id, CancellationToken.None);
+
+        Assert.Equal("aborted", result.Status);
+        Assert.False(File.Exists(Path.Combine(repo.RepoDir, ".git", "MERGE_HEAD")));
+
+        using var ctx = db.CreateContext();
+        var wt = ctx.Worktrees.Single(w => w.TaskId == task.Id);
+        Assert.Equal(WorktreeState.Active, wt.State);
+    }
+
     [Fact]
     public async Task MergeAsync_LeaveConflicts_DoesNotAbortAndReturnsConflictFiles()
     {