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/ClaudeDoDbContext.cs

@@ -43,6 +43,7 @@ public class ClaudeDoDbContext : DbContext
     public DbSet<TaskEntity> Tasks => Set<TaskEntity>();
     public DbSet<TaskDependencyEntity> TaskDependencies => Set<TaskDependencyEntity>();
     public DbSet<ListEntity> Lists => Set<ListEntity>();
+    public DbSet<ListFolderEntity> ListFolders => Set<ListFolderEntity>();
     public DbSet<ListConfigEntity> ListConfigs => Set<ListConfigEntity>();
     public DbSet<WorktreeEntity> Worktrees => Set<WorktreeEntity>();
     public DbSet<TaskRunEntity> TaskRuns => Set<TaskRunEntity>();