feat(tasks): offer Mark as Idle for cancelled tasks

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/ClaudeDo.Worker.Tests/Hub/SetTaskStatusHubTests.cs

@@ -76,6 +76,38 @@ public sealed class SetTaskStatusHubTests : IDisposable
         Assert.Equal(TaskStatus.Cancelled, task!.Status);
     }
 
+    [Fact]
+    public async Task SetTaskStatus_Idle_OnCancelledTask_ClearsTimestampsAndKeepsWorktree()
+    {
+        var taskId = await SeedTaskAsync(TaskStatus.Cancelled);
+        await using (var seed = _db.CreateContext())
+        {
+            var t = (await seed.Tasks.FindAsync(taskId))!;
+            t.StartedAt = DateTime.UtcNow.AddMinutes(-5);
+            t.FinishedAt = DateTime.UtcNow;
+            t.Result = "stale";
+            await seed.SaveChangesAsync();
+            await new WorktreeRepository(seed).AddAsync(new WorktreeEntity
+            {
+                TaskId = taskId, Path = "/tmp/x", BranchName = "claudedo/x",
+                BaseCommit = "deadbeef", State = WorktreeState.Active, CreatedAt = DateTime.UtcNow,
+            });
+        }
+        var hub = CreateHub();
+
+        await hub.SetTaskStatus(taskId, "Idle");
+
+        await using var ctx = _db.CreateContext();
+        var task = await ctx.Tasks.AsNoTracking().SingleAsync(t => t.Id == taskId);
+        Assert.Equal(TaskStatus.Idle, task.Status);
+        Assert.Null(task.StartedAt);
+        Assert.Null(task.FinishedAt);
+        Assert.Null(task.Result);
+        var wt = await ctx.Worktrees.AsNoTracking().SingleAsync(w => w.TaskId == taskId);
+        Assert.Equal(WorktreeState.Active, wt.State);
+        Assert.Equal("claudedo/x", wt.BranchName);
+    }
+
     [Fact]
     public async Task SetTaskStatus_Missing_Throws()
     {