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/Queue/RunCancellationRegistryTests.cs

@@ -1,4 +1,5 @@
 using ClaudeDo.Worker.Queue;
+using Microsoft.Extensions.Logging.Abstractions;
 
 namespace ClaudeDo.Worker.Tests.Queue;
 
@@ -7,9 +8,9 @@ public sealed class RunCancellationRegistryTests
     [Fact]
     public void TryCancel_RegisteredTask_CancelsAndReturnsTrue()
     {
-        var sut = new RunCancellationRegistry();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         using var cts = new CancellationTokenSource();
-        sut.Register("t1", cts);
+        Assert.True(sut.Register("t1", cts));
 
         Assert.True(sut.TryCancel("t1"));
         Assert.True(cts.IsCancellationRequested);
@@ -18,30 +19,50 @@ public sealed class RunCancellationRegistryTests
     [Fact]
     public void TryCancel_UnknownTask_ReturnsFalse()
     {
-        var sut = new RunCancellationRegistry();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         Assert.False(sut.TryCancel("nope"));
     }
 
+    [Fact]
+    public void Register_SecondCallForSameTask_RefusesAndKeepsFirstRegistration()
+    {
+        // A double-dispatch (e.g. RunNow racing the queue picker for the same task) must not
+        // let the second registration silently clobber the first — that would leave CancelAsync
+        // pointing at the wrong (or already-finished) CTS.
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
+        using var first = new CancellationTokenSource();
+        using var second = new CancellationTokenSource();
+
+        Assert.True(sut.Register("t1", first));
+        Assert.False(sut.Register("t1", second));
+
+        Assert.True(sut.TryCancel("t1"));
+        Assert.True(first.IsCancellationRequested);
+        Assert.False(second.IsCancellationRequested);
+    }
+
     [Fact]
     public void Unregister_RemovesOnlyTheGivenRegistration()
     {
-        var sut = new RunCancellationRegistry();
-        using var stale = new CancellationTokenSource();
-        using var current = new CancellationTokenSource();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
+        using var first = new CancellationTokenSource();
 
-        sut.Register("t1", stale);
-        sut.Register("t1", current); // re-run replaced the registration
-        sut.Unregister("t1", stale); // late cleanup of the old slot must not evict the new one
+        sut.Register("t1", first);
+        sut.Unregister("t1", first);
+
+        // Slot is free again once the run that held it cleans up properly, so a genuine
+        // sequential re-run can register a fresh CTS under the same task id.
+        using var second = new CancellationTokenSource();
+        Assert.True(sut.Register("t1", second));
 
         Assert.True(sut.TryCancel("t1"));
-        Assert.True(current.IsCancellationRequested);
-        Assert.False(stale.IsCancellationRequested);
+        Assert.True(second.IsCancellationRequested);
     }
 
     [Fact]
     public void TryCancel_DisposedCts_ReturnsFalse()
     {
-        var sut = new RunCancellationRegistry();
+        var sut = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
         var cts = new CancellationTokenSource();
         sut.Register("t1", cts);
         cts.Dispose();