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/BatchMcpToolsTests.cs

@@ -201,6 +201,7 @@ public sealed class BatchMcpToolsTests : IDisposable
         Assert.True(found.Found);
         Assert.NotNull(found.Task);
         Assert.Equal(task.Id, found.Task!.Id);
+        Assert.Equal(task.Number, found.Task!.Number);
         Assert.Null(found.TaskFull);
     }
 
@@ -220,6 +221,7 @@ public sealed class BatchMcpToolsTests : IDisposable
         Assert.Null(found.Task);
         Assert.NotNull(found.TaskFull);
         Assert.Equal("the full description", found.TaskFull!.Description);
+        Assert.Equal(task.Number, found.TaskFull!.Number);
     }
 
     [Fact]