feat(attachments): MCP tools to attach/list/remove task files

AttachmentMcpTools exposes add_task_attachment (text or base64),
list_task_attachments, and remove_task_attachment on the external MCP
endpoint, so an agent can prepare reference files (plans, scripts) on a task
that will run later. Re-attaching the same name overwrites; add/remove refuse
on a running task.

src/ClaudeDo.Worker/External/AttachmentMcpTools.cs

@@ -0,0 +1,119 @@
+using System.ComponentModel;
+using System.Text;
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Hub;
+using ModelContextProtocol.Server;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.External;
+
+public sealed record AttachmentDto(string FileName, long ByteSize, DateTime CreatedAt);
+
+[McpServerToolType]
+public sealed class AttachmentMcpTools
+{
+    private readonly TaskRepository _tasks;
+    private readonly TaskAttachmentRepository _attachments;
+    private readonly AttachmentStore _store;
+    private readonly HubBroadcaster _broadcaster;
+
+    public AttachmentMcpTools(
+        TaskRepository tasks,
+        TaskAttachmentRepository attachments,
+        AttachmentStore store,
+        HubBroadcaster broadcaster)
+    {
+        _tasks = tasks;
+        _attachments = attachments;
+        _store = store;
+        _broadcaster = broadcaster;
+    }
+
+    [McpServerTool, Description(
+        "Attach a read-only reference file to a task. These files are handed to the agent at run time, " +
+        "making them useful to prepare context for a task that will run later (e.g. plans, scripts, specs). " +
+        "Pass textContent for plain-text files (plans, markdown, scripts). " +
+        "Pass base64Content only for binary files (images, archives). Exactly one of the two must be provided. " +
+        "Re-attaching a file with the same fileName overwrites the previous version. " +
+        "Refuses if the task is currently Running — cancel it first.")]
+    public async Task<AttachmentDto> AddTaskAttachment(
+        string taskId,
+        string fileName,
+        string? textContent = null,
+        string? base64Content = null,
+        CancellationToken ct = default)
+    {
+        var task = await _tasks.GetByIdAsync(taskId, ct)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status == TaskStatus.Running)
+            throw new InvalidOperationException("Cannot add an attachment to a running task. Cancel it first.");
+
+        if (textContent is null == base64Content is null)
+            throw new InvalidOperationException(
+                "Exactly one of textContent or base64Content must be provided, not both and not neither.");
+
+        byte[] bytes;
+        if (textContent is not null)
+        {
+            bytes = Encoding.UTF8.GetBytes(textContent);
+        }
+        else
+        {
+            try { bytes = Convert.FromBase64String(base64Content!); }
+            catch (FormatException ex)
+            {
+                throw new InvalidOperationException("base64Content is not valid Base64.", ex);
+            }
+        }
+
+        using var ms = new MemoryStream(bytes);
+        var byteSize = await _store.SaveAsync(taskId, fileName, ms, ct);
+
+        var existing = await _attachments.GetAsync(taskId, fileName, ct);
+        if (existing is not null)
+        {
+            existing.ByteSize = byteSize;
+            await _attachments.UpdateAsync(existing, ct);
+        }
+        else
+        {
+            await _attachments.AddAsync(new TaskAttachmentEntity
+            {
+                Id = Guid.NewGuid().ToString(),
+                TaskId = taskId,
+                FileName = fileName,
+                ByteSize = byteSize,
+                CreatedAt = DateTime.UtcNow,
+            }, ct);
+        }
+
+        await _broadcaster.TaskUpdated(taskId);
+        return new AttachmentDto(fileName, byteSize, existing?.CreatedAt ?? DateTime.UtcNow);
+    }
+
+    [McpServerTool, Description("List all attachments on a task (fileName, byteSize, createdAt).")]
+    public async Task<IReadOnlyList<AttachmentDto>> ListTaskAttachments(
+        string taskId, CancellationToken ct = default)
+    {
+        var rows = await _attachments.ListByTaskIdAsync(taskId, ct);
+        return rows.Select(r => new AttachmentDto(r.FileName, r.ByteSize, r.CreatedAt)).ToList();
+    }
+
+    [McpServerTool, Description(
+        "Remove a single attachment from a task. Deletes both the file on disk and the database record. " +
+        "Refuses if the task is currently Running — cancel it first.")]
+    public async Task RemoveTaskAttachment(
+        string taskId, string fileName, CancellationToken ct = default)
+    {
+        var task = await _tasks.GetByIdAsync(taskId, ct)
+            ?? throw new InvalidOperationException($"Task {taskId} not found.");
+        if (task.Status == TaskStatus.Running)
+            throw new InvalidOperationException("Cannot remove an attachment from a running task. Cancel it first.");
+
+        _store.DeleteFile(taskId, fileName);
+        await _attachments.DeleteAsync(taskId, fileName, ct);
+        await _broadcaster.TaskUpdated(taskId);
+    }
+}

src/ClaudeDo.Worker/Program.cs

@@ -256,6 +256,9 @@ if (cfg.ExternalMcpPort > 0)
     externalBuilder.Services.AddScoped<AgentMcpTools>();
     externalBuilder.Services.AddScoped<LifecycleMcpTools>();
     externalBuilder.Services.AddScoped<AppSettingsMcpTools>();
+    externalBuilder.Services.AddSingleton(app.Services.GetRequiredService<AttachmentStore>());
+    externalBuilder.Services.AddScoped<TaskAttachmentRepository>();
+    externalBuilder.Services.AddScoped<AttachmentMcpTools>();
     externalBuilder.Services.AddMcpServer()
         .WithHttpTransport()
         .WithTools<ExternalMcpService>()
@@ -264,7 +267,8 @@ if (cfg.ExternalMcpPort > 0)
         .WithTools<RunHistoryMcpTools>()
         .WithTools<AgentMcpTools>()
         .WithTools<LifecycleMcpTools>()
-        .WithTools<AppSettingsMcpTools>();
+        .WithTools<AppSettingsMcpTools>()
+        .WithTools<AttachmentMcpTools>();
     externalBuilder.WebHost.UseUrls($"http://127.0.0.1:{cfg.ExternalMcpPort}");
 
     externalApp = externalBuilder.Build();