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.Worker/StringExtensions.cs

@@ -0,0 +1,7 @@
+namespace ClaudeDo.Worker;
+
+internal static class StringExtensions
+{
+    /// <summary>Returns null for null/whitespace input, otherwise the original string.</summary>
+    public static string? NullIfBlank(this string? s) => string.IsNullOrWhiteSpace(s) ? null : s;
+}