feat(worker): surface task numbers in MCP tool return payloads

Adds Number alongside every task id in External/'s DTOs -- the two
central mappers (ToDto/ToRefDto -> TaskDto/TaskRefDto) plus every
DTO that carries a bare task id and bypasses them (batch results,
queue state, wait-for-change, config, attachments, handoff,
lifecycle, merge-preview-set, worktree list). Input resolution
(#123 as an argument) stays for slice 3.

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -217,6 +217,18 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Equal("new title", dto.Title);
     }
 
+    [Fact]
+    public async Task GetTask_ReturnsTaskNumber()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId);
+        var sut = BuildSut(CreateQueue());
+
+        var dto = await sut.GetTask(task.Id, CancellationToken.None);
+
+        Assert.Equal(task.Number, dto.Number);
+    }
+
     [Fact]
     public async Task GetTask_ReturnsFullDtoIncludingDescription()
     {
@@ -1281,6 +1293,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Null(result.TasksFull);
         Assert.Single(result.Tasks!);
         Assert.Equal(task.Id, result.Tasks![0].Id);
+        Assert.Equal(task.Number, result.Tasks![0].Number);
     }
 
     [Fact]
@@ -1299,6 +1312,7 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.NotNull(result.TasksFull);
         Assert.Single(result.TasksFull!);
         Assert.Equal("the full description", result.TasksFull![0].Description);
+        Assert.Equal(task.Number, result.TasksFull![0].Number);
     }
 
     // ── MergeTask allowWaitingForReview ───────────────────────────────────────
@@ -2341,6 +2355,19 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Equal("dotnet build", fakeVerify.CapturedCommand);
     }
 
+    [Fact]
+    public async Task AddTask_ReturnsAllocatedTaskNumber()
+    {
+        var listId = await SeedListAsync();
+        var sut = NewService();
+
+        var dto = await sut.AddTask(listId, "t", cancellationToken: CancellationToken.None);
+
+        var loaded = await _tasks.GetByIdAsync(dto.Task.Id);
+        Assert.True(dto.Task.Number > 0);
+        Assert.Equal(loaded!.Number, dto.Task.Number);
+    }
+
     // ── AddTask model override ────────────────────────────────────────────────
 
     [Fact]