feat(worker): expose usage/model-usage hub surface and persist run model

Adds GetUsageSnapshot/GetModelUsage/GetTaskUsage to WorkerHub (backed by a
shared UsageSnapshotBuilder), a UsageUpdated broadcast fired after every
UsageMonitorService poll cycle, and records the resolved model on each
task_runs row so per-model/per-task usage can be reported from history.

tests/ClaudeDo.Worker.Tests/Runner/RunModelPersistenceTests.cs

@@ -0,0 +1,105 @@
+using ClaudeDo.Data;
+using ClaudeDo.Data.Git;
+using ClaudeDo.Data.Models;
+using ClaudeDo.Data.Repositories;
+using ClaudeDo.Worker.Config;
+using ClaudeDo.Worker.Hub;
+using ClaudeDo.Worker.Runner;
+using ClaudeDo.Worker.Tests.Infrastructure;
+using Microsoft.Extensions.Logging.Abstractions;
+using TaskStatus = ClaudeDo.Data.Models.TaskStatus;
+using Xunit;
+
+namespace ClaudeDo.Worker.Tests.Runner;
+
+/// Verifies TaskRunner persists the resolved model (task -> list -> AppSettings.DefaultModel)
+/// onto the task_runs row it creates, since the hub's usage-by-model report reads it back from there.
+public sealed class RunModelPersistenceTests : IDisposable
+{
+    private readonly DbFixture _db = new();
+    private readonly string _tempDir;
+    private readonly WorkerConfig _cfg;
+
+    public RunModelPersistenceTests()
+    {
+        _tempDir = Path.Combine(Path.GetTempPath(), $"cd_runmodel_{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 TaskRunner BuildRunner()
+    {
+        var dbFactory = _db.CreateFactory();
+        var state = TaskStateServiceBuilder.Build(dbFactory).State;
+        var wt = new WorktreeManager(new GitService(), dbFactory, _cfg, NullLogger<WorktreeManager>.Instance);
+        var fake = new FakeClaudeProcess((_, _, _, _, _) =>
+            Task.FromResult(new RunResult { ExitCode = 0, ResultMarkdown = "ok" }));
+        return new TaskRunner(fake, dbFactory, new HubBroadcaster(new CapturingHubContext()), wt,
+            new ClaudeArgsBuilder(), _cfg, NullLogger<TaskRunner>.Instance, state, new TaskRunTokenRegistry(),
+            new AttachmentStore(), new FakeSessionSkillSeeder());
+    }
+
+    private async Task SeedAsync(string? taskModel, string? listModel)
+    {
+        using var ctx = _db.CreateContext();
+        ctx.Lists.Add(new ListEntity { Id = "l1", Name = "L", WorkingDir = null, CreatedAt = DateTime.UtcNow });
+        if (listModel is not null)
+            ctx.ListConfigs.Add(new ListConfigEntity { ListId = "l1", Model = listModel });
+        ctx.Tasks.Add(new TaskEntity
+        {
+            Id = "t1", ListId = "l1", Title = "Task", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow,
+            Model = taskModel,
+        });
+        await ctx.SaveChangesAsync();
+    }
+
+    private async Task<string?> RunAndGetPersistedModelAsync()
+    {
+        var runner = BuildRunner();
+        using (var ctx = _db.CreateContext())
+            await runner.RunAsync((await new TaskRepository(ctx).GetByIdAsync("t1"))!, "slot-1", CancellationToken.None);
+
+        using var readCtx = _db.CreateContext();
+        var run = await new TaskRunRepository(readCtx).GetLatestByTaskIdAsync("t1");
+        return run?.Model;
+    }
+
+    [Fact]
+    public async Task Task_level_model_override_is_persisted_on_the_run()
+    {
+        await SeedAsync(taskModel: "opus", listModel: "haiku");
+
+        var persisted = await RunAndGetPersistedModelAsync();
+
+        Assert.Equal("opus", persisted);
+    }
+
+    [Fact]
+    public async Task List_default_model_is_persisted_when_task_has_no_override()
+    {
+        await SeedAsync(taskModel: null, listModel: "haiku");
+
+        var persisted = await RunAndGetPersistedModelAsync();
+
+        Assert.Equal("haiku", persisted);
+    }
+
+    [Fact]
+    public async Task Global_default_model_is_persisted_when_task_and_list_have_no_override()
+    {
+        await SeedAsync(taskModel: null, listModel: null);
+
+        string globalDefault;
+        using (var ctx = _db.CreateContext())
+        {
+            var settings = await new AppSettingsRepository(ctx).GetAsync();
+            globalDefault = settings.DefaultModel;
+        }
+
+        var persisted = await RunAndGetPersistedModelAsync();
+
+        Assert.Equal(globalDefault, persisted);
+    }
+}