feat(usage): add per-task token usage tracking

Extract token usage from Claude Code's stream-json output in real-time
during task execution, replacing the inaccurate global JSONL log scanner.

- New `task_usage` table: tracks (task_id, provider, model) level usage
- Agent SDK: parse `message.usage` from assistant messages, accumulate
  per-model and return in Result
- Daemon: convert agent usage to entries, send with CompleteTask
- Server: store usage on task completion, expose workspace-level
  aggregation APIs (GET /api/usage/daily, GET /api/usage/summary)

server/migrations/032_task_usage.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS task_usage;