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/Repositories/ListRepository.cs

@@ -68,6 +68,58 @@ public sealed class ListRepository
         await _context.SaveChangesAsync(ct);
     }
 
+    // ---- Sidebar folders (list_folders) ----
+    // Folders live here rather than in their own repository because they exist only as a grouping
+    // of these lists — every caller that touches one already holds a ListRepository.
+
+    public async Task<List<ListFolderEntity>> GetFoldersAsync(CancellationToken ct = default)
+    {
+        return await _context.ListFolders
+            .OrderBy(f => f.SortOrder).ThenBy(f => f.CreatedAt)
+            .ToListAsync(ct);
+    }
+
+    public async Task AddFolderAsync(ListFolderEntity folder, CancellationToken ct = default)
+    {
+        folder.SortOrder = await _context.ListFolders.CountAsync(ct);
+        _context.ListFolders.Add(folder);
+        await _context.SaveChangesAsync(ct);
+    }
+
+    /// <summary>Patches just the two editable fields — a whole-entity Update would need
+    /// <c>CreatedAt</c>/<c>SortOrder</c>/<c>IsCollapsed</c> carried along and silently reset
+    /// whichever the caller forgot (see the SetConfigAsync note in CLAUDE.md).</summary>
+    public async Task RenameFolderAsync(string folderId, string name, string? icon, CancellationToken ct = default)
+    {
+        await _context.ListFolders
+            .Where(f => f.Id == folderId)
+            .ExecuteUpdateAsync(s => s
+                .SetProperty(f => f.Name, name)
+                .SetProperty(f => f.Icon, icon), ct);
+    }
+
+    /// <summary>Deletes the folder only — its lists survive and fall back to the top level via
+    /// the ON DELETE SET NULL on <c>lists.folder_id</c>.</summary>
+    public async Task DeleteFolderAsync(string folderId, CancellationToken ct = default)
+    {
+        await _context.ListFolders.Where(f => f.Id == folderId).ExecuteDeleteAsync(ct);
+    }
+
+    public async Task SetFolderCollapsedAsync(string folderId, bool collapsed, CancellationToken ct = default)
+    {
+        await _context.ListFolders
+            .Where(f => f.Id == folderId)
+            .ExecuteUpdateAsync(s => s.SetProperty(f => f.IsCollapsed, collapsed), ct);
+    }
+
+    /// <summary>Moves a list into <paramref name="folderId"/>, or back to the top level when null.</summary>
+    public async Task SetListFolderAsync(string listId, string? folderId, CancellationToken ct = default)
+    {
+        await _context.Lists
+            .Where(l => l.Id == listId)
+            .ExecuteUpdateAsync(s => s.SetProperty(l => l.FolderId, folderId), ct);
+    }
+
     public async Task<ListConfigEntity?> GetConfigAsync(string listId, CancellationToken ct = default)
     {
         return await _context.ListConfigs.FirstOrDefaultAsync(c => c.ListId == listId, ct);