feat(data): list_folders table plus lists.folder_id

Sidebar folders get their own table rather than a flag on lists, so the
places that enumerate lists (MCP tools, move-to-list, queue picker, repo
import) need no folder filtering. ON DELETE SET NULL keeps a folder's
lists alive when the folder goes.

src/ClaudeDo.Data/Models/ListFolderEntity.cs

@@ -0,0 +1,20 @@
+namespace ClaudeDo.Data.Models;
+
+/// <summary>
+/// A sidebar folder grouping user lists. Deliberately its own table rather than a flag on
+/// <see cref="ListEntity"/>: every place that enumerates lists (MCP list_task_lists, "move to
+/// list", the queue picker, repo import) would otherwise have to filter folders out.
+/// One level only — a folder holds lists, never other folders.
+/// </summary>
+public sealed class ListFolderEntity
+{
+    public required string Id { get; init; }
+    public required string Name { get; set; }
+    public required DateTime CreatedAt { get; init; }
+    // Icon key into the UI's icon library, same set as ListEntity.Icon; null = default folder icon.
+    public string? Icon { get; set; }
+    public int SortOrder { get; set; }
+    // Sidebar expand/collapse state. Lives here rather than in ui.config.json because the row is
+    // already being read for the folder itself.
+    public bool IsCollapsed { get; set; }
+}