refactor(usage): drop the transcript aggregation path in favour of TokenTracker

tests/ClaudeDo.Worker.Tests/Infrastructure/FakeTranscriptUsageReader.cs

@@ -9,10 +9,6 @@ public sealed class FakeTranscriptUsageReader : ITranscriptUsageReader
 
     public void SetTotals(string sessionId, SessionUsageTotals totals) => _totalsBySession[sessionId] = totals;
 
-    public Task<IReadOnlyList<UsageAggregateRow>> ReadAsync(
-        DateOnly start, DateOnly end, CancellationToken ct = default) =>
-        Task.FromResult<IReadOnlyList<UsageAggregateRow>>(Array.Empty<UsageAggregateRow>());
-
     public Task<SessionUsageTotals?> ReadSessionTotalsAsync(string sessionId, CancellationToken ct = default) =>
         Task.FromResult(_totalsBySession.TryGetValue(sessionId, out var totals) ? totals : null);
 }