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.

tests/ClaudeDo.Worker.Tests/Infrastructure/StaticTokenAuthProvider.cs

@@ -0,0 +1,16 @@
+using ClaudeDo.Worker.Online.Interfaces;
+
+namespace ClaudeDo.Worker.Online;
+
+/// <summary>
+/// Test double: an <see cref="IOnlineAuthProvider"/> that returns a fixed token supplied at
+/// construction. Production uses <c>ZitadelAuthProvider</c>.
+/// </summary>
+public sealed class StaticTokenAuthProvider(string? token = null) : IOnlineAuthProvider
+{
+    public Task<string?> GetAccessTokenAsync(CancellationToken ct = default)
+        => Task.FromResult(token);
+
+    public Task<string?> GetAccessTokenAsync(bool forceRefresh, CancellationToken ct = default)
+        => Task.FromResult(token);
+}