Add seven best-effort batch variants of the single-entity external MCP
tools: batch_get_tasks, batch_add_tasks, batch_update_task_status,
batch_cancel_tasks, batch_delete_tasks, batch_set_my_day, and
batch_cleanup_task_worktrees. Each loops the existing ExternalMcpService
methods sequentially (scoped DbContext is not thread-safe), returns a
per-item result array so a failing item never aborts the rest, and
rejects empty or over-100-item batches. Merge/review stay single-task.
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