Background task notifications can now be coalesced

What

• A new flag tengu_swirling_meteor controls whether task-stop notifications from background agents get coalesced (merged together) during a session's live loop.
• Print/headless mode gained a coalesceTaskNotifications option and a new print_task_notification_coalesce telemetry event, tracking how much held completion text and workload is batched.
• The session's message-queue engine gained a coalesceTaskNotifications callback: when enabled, a queued task-notification whose text fits under a length budget is merged into the pending turn instead of being sent out as a separate message. This path is currently gated off by default.

Why

When several background tasks (like background agents or shell commands) finish around the same time, Claude Code can now group their completion notifications into one update instead of interrupting the session repeatedly.