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.

tests/ClaudeDo.Data.Tests/TaskPromptComposerTests.cs

@@ -42,4 +42,44 @@ public class TaskPromptComposerTests
     {
         Assert.Equal("Just a title", TaskPromptComposer.Compose("Just a title", null, System.Array.Empty<(string, bool)>()));
     }
+
+    [Fact]
+    public void Attachment_section_present_when_paths_given()
+    {
+        var result = TaskPromptComposer.Compose(
+            "Title", null, System.Array.Empty<(string, bool)>(),
+            new[] { "/a/b/file1.txt", "/a/b/file2.txt" });
+
+        Assert.Contains("## Reference files", result);
+        Assert.Contains("- /a/b/file1.txt", result);
+        Assert.Contains("- /a/b/file2.txt", result);
+    }
+
+    [Fact]
+    public void Attachment_section_absent_when_null()
+    {
+        var result = TaskPromptComposer.Compose("Title", null, System.Array.Empty<(string, bool)>(), null);
+        Assert.DoesNotContain("Reference files", result);
+    }
+
+    [Fact]
+    public void Attachment_section_absent_when_empty()
+    {
+        var result = TaskPromptComposer.Compose("Title", null, System.Array.Empty<(string, bool)>(),
+            System.Array.Empty<string>());
+        Assert.DoesNotContain("Reference files", result);
+    }
+
+    [Fact]
+    public void Attachment_paths_order_preserved()
+    {
+        var paths = new[] { "/z/last.txt", "/a/first.txt", "/m/middle.txt" };
+        var result = TaskPromptComposer.Compose("Title", null, System.Array.Empty<(string, bool)>(), paths);
+
+        var idxZ = result.IndexOf("/z/last.txt", StringComparison.Ordinal);
+        var idxA = result.IndexOf("/a/first.txt", StringComparison.Ordinal);
+        var idxM = result.IndexOf("/m/middle.txt", StringComparison.Ordinal);
+
+        Assert.True(idxZ < idxA && idxA < idxM, "Paths should appear in the original order.");
+    }
 }