refactor: extract interfaces to Interfaces folders and consolidate filters

Move interface declarations into per-area Interfaces/ subfolders, merge the
small task-list filter classes into StatusFilter/SmartFlagFilter, and simplify
related services, converters and hub DTO handling.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Installer/Core/Interfaces/IInstallStep.cs

@@ -0,0 +1,20 @@
+namespace ClaudeDo.Installer.Core;
+
+public interface IInstallStep
+{
+    string Name { get; }
+    Task<StepResult> ExecuteAsync(InstallContext ctx, IProgress<string> progress, CancellationToken ct);
+}
+
+public sealed class StepResult
+{
+    public bool Success { get; init; }
+    public string? ErrorMessage { get; init; }
+
+    public static StepResult Ok() => new() { Success = true };
+    public static StepResult Fail(string error) => new() { Success = false, ErrorMessage = error };
+}
+
+public enum StepStatus { Pending, Running, Done, Failed, Skipped }
+
+public sealed record StepProgress(string StepName, StepStatus Status, string? Message = null);

src/ClaudeDo.Installer/Core/Interfaces/IInstallerPage.cs

@@ -0,0 +1,16 @@
+using System.Windows.Controls;
+
+namespace ClaudeDo.Installer.Core;
+
+public interface IInstallerPage
+{
+    string Title { get; }
+    string Icon { get; }
+    int Order { get; }
+    bool ShowInWizard { get; }
+    bool ShowInSettings { get; }
+    UserControl View { get; }
+    Task LoadAsync();
+    Task ApplyAsync();
+    bool Validate();
+}