* feat(issues): display token usage per issue in detail sidebar
Add a new "Token usage" section to the issue detail right sidebar that
shows aggregated input/output tokens, cache tokens, and run count across
all tasks for the issue. Backed by a new SQL query and API endpoint.
* fix(db): add index on agent_task_queue(issue_id) for usage queries
The GetIssueUsageSummary query joins agent_task_queue filtered by
issue_id across all statuses. The existing partial index (migration 022)
only covers queued/dispatched rows, so completed tasks require a
sequential scan. Add a general index to prevent performance degradation
as task volume grows.
