feat(worker): accept #123 task numbers as MCP tool input

TaskIdResolver resolves a #123/bare-123 taskId parameter to its GUID
before any lookup, across every External/ MCP tool that takes a task
id, including the batch tools' id arrays (via delegation to the
already-resolving single-entity methods) and update_task's
dependsOnTaskId (empty string still passes through unchanged as the
clear-link sentinel). An unknown number throws a clear error instead
of a silent null. McpToolDocs.TaskNumberHint tells the agent to refer
to tasks as #<number> when reporting to the user, added to the
description of get_task, list_tasks, add_task, update_task_status and
review_task.

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

@@ -229,6 +229,41 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Equal(task.Number, dto.Number);
     }
 
+    [Fact]
+    public async Task GetTask_ByHashNumber_ResolvesToTask()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId);
+        var sut = BuildSut(CreateQueue());
+
+        var dto = await sut.GetTask($"#{task.Number}", CancellationToken.None);
+
+        Assert.Equal(task.Id, dto.Id);
+    }
+
+    [Fact]
+    public async Task GetTask_ByBareNumber_ResolvesToTask()
+    {
+        var listId = await SeedListAsync();
+        var task = await SeedTaskAsync(listId);
+        var sut = BuildSut(CreateQueue());
+
+        var dto = await sut.GetTask(task.Number.ToString(), CancellationToken.None);
+
+        Assert.Equal(task.Id, dto.Id);
+    }
+
+    [Fact]
+    public async Task GetTask_UnknownNumber_ThrowsWithNumberInMessage()
+    {
+        var sut = BuildSut(CreateQueue());
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            sut.GetTask("#999999", CancellationToken.None));
+
+        Assert.Contains("999999", ex.Message);
+    }
+
     [Fact]
     public async Task GetTask_ReturnsFullDtoIncludingDescription()
     {