fix(worker): claim Running before creating run resources

TaskRunner.RunAsync created the worktree (PrepareRunDirectoryAsync)
before claiming Running via StartRunningAsync. RunNow dispatches by
task id with no atomic claim of their own, so a Queued task racing
the queue picker's atomic SQL claim could hit WorktreeManager's
branch-collision self-heal, which force-removes and recreates the
winner's live worktree mid-run.

Move the claim before any resource creation and bail out immediately
when it's rejected. OverrideSlotService.RunNow also fast-rejects a
task already Running in the DB (defense in depth). RunCancellationRegistry
now refuses (and logs) a double registration instead of silently
overwriting the first CTS, so a losing dispatch's cleanup can no
longer unregister the winner's cancellation token.

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

@@ -25,7 +25,7 @@ public static class TaskStateServiceBuilder
         var hub = new CapturingHubContext();
         var broadcaster = new HubBroadcaster(hub);
         var waker = new CountingQueueWaker();
-        var runCancels = new RunCancellationRegistry();
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
 
         TaskStateService? state = null;
         var chain = new PlanningChainCoordinator(dbFactory, () => state!);