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.

src/ClaudeDo.Worker/Runner/TaskRunner.cs

@@ -318,6 +318,7 @@ public sealed class TaskRunner
             Prompt = prompt,
             LogPath = logPath,
             StartedAt = DateTime.UtcNow,
+            Model = config.Model,
         };
 
         using (var context = _dbFactory.CreateDbContext())