feat(attachments): inject reference files into the run + clean up files on delete

TaskRunner appends attached files (absolute paths) to the run prompt as the
read-only Reference files section. Task and list deletes now remove the
on-disk attachment dir eagerly, and a startup AttachmentOrphanRecovery sweep
drops any attachments/<taskId>/ whose task no longer exists (covers list
cascade and planning-discard paths).

src/ClaudeDo.Worker/Program.cs

@@ -46,8 +46,10 @@ builder.Services.AddDbContextFactory<ClaudeDoDbContext>(opt =>
     opt.UseSqlite($"Data Source={cfg.DbPath}"));
 
 builder.Services.AddSingleton(cfg);
+builder.Services.AddSingleton<AttachmentStore>();
 builder.Services.AddHostedService<StaleTaskRecovery>();
 builder.Services.AddHostedService<OrphanRecovery>();
+builder.Services.AddHostedService<AttachmentOrphanRecovery>();
 builder.Services.AddSignalR().AddJsonProtocol(options =>
 {
     options.PayloadSerializerOptions.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());