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.Worker/Prime/Interfaces/IPrimeScheduleSignal.cs

@@ -1,6 +0,0 @@
-namespace ClaudeDo.Worker.Prime;
-public interface IPrimeScheduleSignal
-{
-    void Signal();
-    CancellationToken CurrentToken { get; }
-}

src/ClaudeDo.Worker/Prime/PrimeScheduleSignal.cs

@@ -1,6 +1,6 @@
 namespace ClaudeDo.Worker.Prime;
 
-public sealed class PrimeScheduleSignal : IPrimeScheduleSignal, IDisposable
+public sealed class PrimeScheduleSignal : IDisposable
 {
     private CancellationTokenSource _cts = new();
     private readonly object _lock = new();

src/ClaudeDo.Worker/Prime/PrimeScheduler.cs

@@ -21,7 +21,7 @@ public sealed class PrimeScheduler : BackgroundService
     private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
     private readonly IPrimeRunner _runner;
     private readonly IPrimeClock _clock;
-    private readonly IPrimeScheduleSignal _signal;
+    private readonly PrimeScheduleSignal _signal;
     private readonly IPrimeBroadcaster _broadcaster;
     private readonly PrimeSchedulerOptions _options;
     private readonly ILogger<PrimeScheduler> _logger;
@@ -30,7 +30,7 @@ public sealed class PrimeScheduler : BackgroundService
         IDbContextFactory<ClaudeDoDbContext> dbFactory,
         IPrimeRunner runner,
         IPrimeClock clock,
-        IPrimeScheduleSignal signal,
+        PrimeScheduleSignal signal,
         IPrimeBroadcaster broadcaster,
         PrimeSchedulerOptions options,
         ILogger<PrimeScheduler> logger)