refactor: collapse single-implementation interfaces

Nine interfaces had exactly one implementation and no test double — they existed
only to be named twice in a DI registration: IFindingsStore, IFindingsStoreLocator,
IPrimeScheduleSignal, IRefineRunner, IWeekReportService, IMergeCoordinator,
IMissionControlPane, IOnlineLoginService, ITaskListFilter. Consumers now depend on
the concrete type; the DTO records that shared those files moved next to their
implementation. IInteractiveLaunchSpecService stays — it carries 54 lines of
contract documentation, which is not ceremony.

IMergeCoordinator in particular had a redundant null object: MergeCoordinator with
a null Handler already no-ops, and every test used the real class with Handler set.

Filtering/ collapses from 8 files to 1. ITaskListFilter and TaskListFilterBase were
a double abstraction over four predicates, with MatchesAsContext => false declared
in both. SmartFlagFilter also compiled its expression twice (its own _flag plus the
inherited Matches cache) — it now uses the cache.

StaticTokenAuthProvider was in src but production uses ZitadelAuthProvider; it is
a test double, so it moves to the test project. Its own test goes away with it.

src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs

@@ -21,7 +21,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
     private readonly IWorkerClient _worker;
     private readonly IServiceProvider _services;
     private readonly INotesApi _notesApi;
-    private readonly IMergeCoordinator _merge;
+    private readonly MergeCoordinator _merge;
 
     // ── Section view models ───────────────────────────────────────────────────
     public AgentConfigEditorViewModel AgentSettings { get; }
@@ -325,7 +325,7 @@ public sealed partial class DetailsIslandViewModel : ViewModelBase, IDisposable
         IWorkerClient worker,
         IServiceProvider services,
         INotesApi notesApi,
-        IMergeCoordinator merge)
+        MergeCoordinator merge)
     {
         _dbFactory = dbFactory;
         _worker = worker;