feat(tui): fire-and-forget async subagent tasks

[IgorWarzocha]

Summary

• Add fire-and-forget async subagent tasks with parent notifications and status tracking.
• Surface background task state in the TUI and system prompt, including subtask reminders and navigation.
• Add async task limits and async-only permission rules for tighter control.
• <task_tool_agents> system reminder similar to <skills_available> with a list of currently running async subagents.

Config

• experimental.async_task_limit (default 3)
• experimental.async_task_permissions
• permission.task_async

UX

• Sidebar shows background tasks with status.
• Session view renders subtask completion reminders and links to subagent sessions.
• Compaction includes running async tasks to preserve context.

Testing

• I have tested it as thoroughly as I can, but there's only so much I can do myself (my imagination ran out of edge case scenarios). Hope it at least helps as a stub if a similar feature is planned.