fix(worker): honor RunCancellationRegistry.Register's return value at both dispatch sites

Register(taskId, cts) already refuses (and logs) a double-registration, but
both call sites discarded the bool and dispatched anyway under an
unregistered CTS. If the loser then unregistered the winner's CTS during
its own cleanup, TryCancel could silently no-op against a live process.

- OverrideSlotService.StartInSlot now fails RunNow/ContinueTask loudly
  (throws) when it loses the registration race instead of registering
  over — or silently proceeding despite losing to — the queue picker.
- QueueService's picker loop retries registration briefly (the loser's own
  claim-attempt-then-unregister resolves fast) before dispatching; if
  registration never resolves it marks the already-claimed row Failed
  instead of running it unregistered.
- RunCancellationRegistry.Unregister already had compare-and-remove
  semantics (TryRemove(KeyValuePair)), so a loser's cleanup could not have
  removed the winner's CTS once registration correctly failed.

Added regression tests exercising the real registry through both dispatch
paths: RunNow losing the registration race throws without disturbing the
winner, the picker's retry succeeds and TryCancel reaches the live run when
the loser unregisters in time, and the picker fails the task instead of
running unregistered when it never does.

src/ClaudeDo.Worker/Queue/OverrideSlotService.cs

@@ -74,8 +74,14 @@ public sealed class OverrideSlotService
                 throw new InvalidOperationException("override slot busy");
 
             var cts = new CancellationTokenSource();
+            if (!_runCancels.Register(taskId, cts))
+            {
+                cts.Dispose();
+                throw new InvalidOperationException(
+                    $"Task '{taskId}' lost the double-dispatch race to the queue picker; it is already running there.");
+            }
+
             _slot = new QueueSlotState { TaskId = taskId, StartedAt = DateTime.UtcNow, Cts = cts };
-            _runCancels.Register(taskId, cts);
 
             _ = work(cts.Token).ContinueWith(t =>
             {