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/OverrideSlotServiceTests.cs

@@ -0,0 +1,89 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Queue;
+using ClaudeDo.Worker.Runner;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+
+namespace ClaudeDo.Worker.Tests.Queue;
+
+/// OverrideSlotService.RunNow's own precheck — distinct from QueueService.EnsureNotInQueueSlot,
+/// which only guards a task already tracked in its in-memory queue-slot dict. This covers a
+/// task the DB already shows Running (e.g. the queue picker's atomic claim landed) regardless
+/// of local slot bookkeeping. The real race-safety net is TaskRunner.RunAsync claiming Running
+/// before creating any resources; this precheck just fails fast for the common case.
+public sealed class OverrideSlotServiceTests : IDisposable
+{
+    private readonly DbFixture _db = new();
+    private readonly string _tempDir;
+    private readonly WorkerConfig _cfg;
+
+    public OverrideSlotServiceTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"cd_override_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_tempDir);
+        _cfg = new WorkerConfig { SandboxRoot = _tempDir, LogRoot = _tempDir };
+    }
+
+    public void Dispose() { _db.Dispose(); try { Directory.Delete(_tempDir, true); } catch { } }
+
+    private OverrideSlotService BuildService()
+    {
+        var dbFactory = _db.CreateFactory();
+        var state = TaskStateServiceBuilder.Build(dbFactory).State;
+        var wt = new WorktreeManager(new ClaudeDo.Data.Git.GitService(), dbFactory, _cfg, NullLogger<WorktreeManager>.Instance);
+        var runner = new TaskRunner(new FakeClaudeProcess(), dbFactory, new HubBroadcaster(new CapturingHubContext()), wt,
+            new ClaudeArgsBuilder(), _cfg, NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(),
+            new AttachmentStore(), new FakeSessionSkillSeeder(), new FakeTranscriptUsageReader());
+        var runCancels = new RunCancellationRegistry(NullLogger<RunCancellationRegistry>.Instance);
+        return new OverrideSlotService(dbFactory, runner, NullLogger<OverrideSlotService>.Instance, runCancels);
+    }
+
+    [Fact]
+    public async Task RunNow_TaskAlreadyRunningInDb_ThrowsWithoutDispatching()
+    {
+        string listId = Guid.NewGuid().ToString(), taskId = Guid.NewGuid().ToString();
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity
+            {
+                Id = taskId, ListId = listId, Title = "T", Status = TaskStatus.Running,
+                StartedAt = DateTime.UtcNow, CreatedAt = DateTime.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        var service = BuildService();
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(() => service.RunNow(taskId));
+        Assert.Contains("already running", ex.Message);
+        Assert.Null(service.CurrentSlot);
+    }
+
+    [Fact]
+    public async Task RunNow_TaskQueued_DoesNotThrow()
+    {
+        string listId = Guid.NewGuid().ToString(), taskId = Guid.NewGuid().ToString();
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity
+            {
+                Id = taskId, ListId = listId, Title = "T", Status = TaskStatus.Queued,
+                CreatedAt = DateTime.UtcNow,
+            });
+            await ctx.SaveChangesAsync();
+        }
+
+        var service = BuildService();
+
+        // The dispatch itself races with FakeClaudeProcess's immediate completion in the
+        // background, so this only asserts the precheck doesn't reject a claimable task.
+        await service.RunNow(taskId);
+    }
+}