feat(attachments): data layer for task file attachments

TaskAttachmentEntity (+config, cascade FK), TaskAttachmentRepository, and an
AttachmentStore that writes files under ~/.todo-app/attachments/<taskId>/ with
a path-traversal guard and a 5 MB cap. TaskPromptComposer gains an optional
read-only 'Reference files' section. Migration AddTaskAttachments.

src/ClaudeDo.Data/Configuration/TaskAttachmentEntityConfiguration.cs

@@ -0,0 +1,27 @@
+using ClaudeDo.Data.Models;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace ClaudeDo.Data.Configuration;
+
+public class TaskAttachmentEntityConfiguration : IEntityTypeConfiguration<TaskAttachmentEntity>
+{
+    public void Configure(EntityTypeBuilder<TaskAttachmentEntity> builder)
+    {
+        builder.ToTable("task_attachments");
+
+        builder.HasKey(a => a.Id);
+        builder.Property(a => a.Id).HasColumnName("id");
+        builder.Property(a => a.TaskId).HasColumnName("task_id").IsRequired();
+        builder.Property(a => a.FileName).HasColumnName("file_name").IsRequired();
+        builder.Property(a => a.ByteSize).HasColumnName("byte_size").IsRequired();
+        builder.Property(a => a.CreatedAt).HasColumnName("created_at").IsRequired();
+
+        builder.HasOne(a => a.Task)
+            .WithMany()
+            .HasForeignKey(a => a.TaskId)
+            .OnDelete(DeleteBehavior.Cascade);
+
+        builder.HasIndex(a => a.TaskId).HasDatabaseName("idx_task_attachments_task_id");
+    }
+}