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/External/FindingsMcpTools.cs

@@ -11,10 +11,10 @@ public sealed record SaveFindingResult(
 [McpServerToolType]
 public sealed class FindingsMcpTools
 {
-    private readonly IFindingsStore _store;
-    private readonly IFindingsStoreLocator _locator;
+    private readonly FindingsStore _store;
+    private readonly FindingsStoreLocator _locator;
 
-    public FindingsMcpTools(IFindingsStore store, IFindingsStoreLocator locator)
+    public FindingsMcpTools(FindingsStore store, FindingsStoreLocator locator)
     {
         _store = store;
         _locator = locator;