Replace raw ADO.NET implementations with EF Core LINQ queries and
ExecuteUpdate/ExecuteDelete for bulk operations. TaskRepository preserves
FlipAllRunningToFailedAsync(reason) signature and keeps raw SQL for the
atomic queue claim (UPDATE...RETURNING). GetByListAsync alias kept for
backwards compat.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Per-task checklist backend: subtasks table with CASCADE delete,
SubtaskEntity + SubtaskRepository (connection-per-op, async), DI
registration in App and Worker, TaskRunner composes a '## Sub-Tasks'
markdown block into the Claude prompt when subtasks exist.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>