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.

src/ClaudeDo.Worker/External/TaskIdResolver.cs

@@ -0,0 +1,46 @@
+using System.Globalization;
+using ClaudeDo.Data.Repositories;
+
+namespace ClaudeDo.Worker.External;
+
+/// <summary>
+/// Resolves a task-id MCP parameter that may be a display number (<c>#123</c> or bare <c>123</c>)
+/// into the task's GUID. A GUID is never all-digits, so the two forms never collide. Anything
+/// else — including an empty string, which <c>update_task</c>'s dependsOnTaskId uses as a
+/// "clear the link" sentinel — passes through untouched.
+/// </summary>
+internal static class TaskIdResolver
+{
+    public static async Task<string> ResolveAsync(TaskRepository tasks, string idOrNumber, CancellationToken ct = default)
+    {
+        return (await ResolveCoreAsync(tasks, idOrNumber, ct))!;
+    }
+
+    public static async Task<string?> ResolveOptionalAsync(TaskRepository tasks, string? idOrNumber, CancellationToken ct = default)
+    {
+        return idOrNumber is null ? null : await ResolveCoreAsync(tasks, idOrNumber, ct);
+    }
+
+    public static async Task<IReadOnlyList<string>> ResolveManyAsync(
+        TaskRepository tasks, IReadOnlyList<string> idsOrNumbers, CancellationToken ct = default)
+    {
+        var resolved = new string[idsOrNumbers.Count];
+        for (var i = 0; i < idsOrNumbers.Count; i++)
+            resolved[i] = await ResolveAsync(tasks, idsOrNumbers[i], ct);
+        return resolved;
+    }
+
+    private static async Task<string?> ResolveCoreAsync(TaskRepository tasks, string idOrNumber, CancellationToken ct)
+    {
+        if (idOrNumber.Length == 0)
+            return idOrNumber;
+
+        var candidate = idOrNumber[0] == '#' ? idOrNumber[1..] : idOrNumber;
+        if (!int.TryParse(candidate, NumberStyles.None, CultureInfo.InvariantCulture, out var number))
+            return idOrNumber;
+
+        var task = await tasks.GetByNumberAsync(number, ct)
+            ?? throw new InvalidOperationException($"no task with number {number}");
+        return task.Id;
+    }
+}