feat(ui): add merge-helper task selection dialog

tests/ClaudeDo.Ui.Tests/ViewModels/MergeHelperSelectionModalViewModelTests.cs

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+using System.Linq;
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Ui.ViewModels.Modals;
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Ui.Tests.ViewModels;
+
+public class MergeHelperSelectionModalViewModelTests : IDisposable
+{
+    private readonly string _dbPath;
+
+    public MergeHelperSelectionModalViewModelTests()
+    {
+        _dbPath = Path.Combine(Path.GetTempPath(), $"claudedo_mh_test_{Guid.NewGuid():N}.db");
+        using var ctx = NewContext();
+        ctx.Database.EnsureCreated();
+    }
+
+    public void Dispose()
+    {
+        try { File.Delete(_dbPath); } catch { }
+        try { File.Delete(_dbPath + "-wal"); } catch { }
+        try { File.Delete(_dbPath + "-shm"); } catch { }
+    }
+
+    private ClaudeDoDbContext NewContext()
+    {
+        var opts = new DbContextOptionsBuilder<ClaudeDoDbContext>()
+            .UseSqlite($"Data Source={_dbPath}")
+            .Options;
+        return new ClaudeDoDbContext(opts);
+    }
+
+    private sealed class TestDbFactory : IDbContextFactory<ClaudeDoDbContext>
+    {
+        private readonly Func<ClaudeDoDbContext> _create;
+        public TestDbFactory(Func<ClaudeDoDbContext> create) => _create = create;
+        public ClaudeDoDbContext CreateDbContext() => _create();
+    }
+
+    private MergeHelperSelectionModalViewModel BuildVm()
+        => new(new TestDbFactory(NewContext));
+
+    private async Task SeedAllStatusesAsync()
+    {
+        await using var db = NewContext();
+        db.Lists.Add(new ListEntity { Id = "L1", Name = "Work", CreatedAt = DateTime.UtcNow });
+        db.Lists.Add(new ListEntity { Id = "L2", Name = "Home", CreatedAt = DateTime.UtcNow });
+        var order = 0;
+        void Add(string id, string listId, TaskStatus status) =>
+            db.Tasks.Add(new TaskEntity
+            {
+                Id = id, ListId = listId, Title = $"task {id}",
+                Status = status, CreatedAt = DateTime.UtcNow, SortOrder = order++,
+            });
+
+        Add("t-idle", "L1", TaskStatus.Idle);
+        Add("t-queued", "L1", TaskStatus.Queued);
+        Add("t-running", "L1", TaskStatus.Running);
+        Add("t-review", "L1", TaskStatus.WaitingForReview);
+        Add("t-children", "L1", TaskStatus.WaitingForChildren);
+        Add("t-failed", "L1", TaskStatus.Failed);
+        Add("t-done", "L1", TaskStatus.Done);
+        Add("t-cancelled", "L1", TaskStatus.Cancelled);
+        Add("t-other-list", "L2", TaskStatus.Idle);
+        await db.SaveChangesAsync();
+    }
+
+    [Fact]
+    public async Task Load_ExcludesTerminalStatuses_AndTicksActionableByDefault()
+    {
+        await SeedAllStatusesAsync();
+        var vm = BuildVm();
+        vm.Configure(null, null);
+        await vm.LoadAsync();
+
+        Assert.DoesNotContain(vm.Tasks, t => t.Id is "t-done" or "t-cancelled");
+        Assert.Equal(7, vm.Tasks.Count);
+
+        Assert.True(vm.Tasks.Single(t => t.Id == "t-idle").IsSelected);
+        Assert.True(vm.Tasks.Single(t => t.Id == "t-queued").IsSelected);
+        Assert.True(vm.Tasks.Single(t => t.Id == "t-review").IsSelected);
+        Assert.True(vm.Tasks.Single(t => t.Id == "t-failed").IsSelected);
+        Assert.False(vm.Tasks.Single(t => t.Id == "t-running").IsSelected);
+        Assert.False(vm.Tasks.Single(t => t.Id == "t-children").IsSelected);
+    }
+
+    [Fact]
+    public async Task Load_PerListScope_FiltersToThatList()
+    {
+        await SeedAllStatusesAsync();
+        var vm = BuildVm();
+        vm.Configure("L2", "Home");
+        await vm.LoadAsync();
+
+        Assert.Single(vm.Tasks);
+        Assert.Equal("t-other-list", vm.Tasks[0].Id);
+        Assert.Equal("Home", vm.Tasks[0].ListName);
+        Assert.False(vm.IsGlobal);
+        Assert.Contains("Home", vm.ScopeLabel);
+    }
+
+    [Fact]
+    public async Task CanConfirm_FollowsRowSelection()
+    {
+        await SeedAllStatusesAsync();
+        var vm = BuildVm();
+        vm.Configure(null, null);
+        await vm.LoadAsync();
+
+        Assert.True(vm.CanConfirm);
+
+        vm.SelectNoneCommand.Execute(null);
+        Assert.False(vm.CanConfirm);
+        Assert.All(vm.Tasks, t => Assert.False(t.IsSelected));
+
+        vm.Tasks[0].IsSelected = true;      // single row re-enables via PropertyChanged hook
+        Assert.True(vm.CanConfirm);
+
+        vm.SelectAllCommand.Execute(null);
+        Assert.All(vm.Tasks, t => Assert.True(t.IsSelected));
+    }
+
+    [Fact]
+    public async Task Confirm_ReturnsSelectedIds_InRowOrder()
+    {
+        await SeedAllStatusesAsync();
+        var vm = BuildVm();
+        vm.Configure(null, null);
+        await vm.LoadAsync();
+
+        vm.SelectNoneCommand.Execute(null);
+        vm.Tasks.Single(t => t.Id == "t-review").IsSelected = true;
+        vm.Tasks.Single(t => t.Id == "t-idle").IsSelected = true;
+
+        var closed = false;
+        vm.CloseAction = () => closed = true;
+        vm.ConfirmCommand.Execute(null);
+
+        var result = await vm.Result.Task;
+        Assert.NotNull(result);
+        // Row order (SortOrder): t-idle was seeded before t-review.
+        Assert.Equal(new[] { "t-idle", "t-review" }, result);
+        Assert.True(closed);
+    }
+
+    [Fact]
+    public async Task Cancel_ReturnsNull()
+    {
+        await SeedAllStatusesAsync();
+        var vm = BuildVm();
+        vm.Configure(null, null);
+        await vm.LoadAsync();
+
+        var closed = false;
+        vm.CloseAction = () => closed = true;
+        vm.CancelCommand.Execute(null);
+
+        Assert.Null(await vm.Result.Task);
+        Assert.True(closed);
+    }
+
+    [Fact]
+    public async Task Load_NoCandidates_HasTasksFalse_CannotConfirm()
+    {
+        await using (var db = NewContext())
+        {
+            db.Lists.Add(new ListEntity { Id = "L1", Name = "Work", CreatedAt = DateTime.UtcNow });
+            db.Tasks.Add(new TaskEntity { Id = "t-done", ListId = "L1", Title = "done", Status = TaskStatus.Done, CreatedAt = DateTime.UtcNow });
+            await db.SaveChangesAsync();
+        }
+
+        var vm = BuildVm();
+        vm.Configure("L1", "Work");
+        await vm.LoadAsync();
+
+        Assert.False(vm.HasTasks);
+        Assert.False(vm.CanConfirm);
+    }
+}