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/Online/StaticTokenAuthProviderTests.cs

@@ -1,30 +0,0 @@
-using ClaudeDo.Worker.Online;
-
-namespace ClaudeDo.Worker.Tests.Online;
-
-public sealed class StaticTokenAuthProviderTests
-{
-    [Fact]
-    public async Task WithToken_Returns_Token()
-    {
-        var provider = new StaticTokenAuthProvider("my-token");
-        var result = await provider.GetAccessTokenAsync();
-        Assert.Equal("my-token", result);
-    }
-
-    [Fact]
-    public async Task WithNull_Returns_Null()
-    {
-        var provider = new StaticTokenAuthProvider(null);
-        var result = await provider.GetAccessTokenAsync();
-        Assert.Null(result);
-    }
-
-    [Fact]
-    public async Task Default_Returns_Null()
-    {
-        var provider = new StaticTokenAuthProvider();
-        var result = await provider.GetAccessTokenAsync();
-        Assert.Null(result);
-    }
-}