Internal upgrade — public tool surface (agent_spawn, agent_swarm, …) unchanged.
The mailbox primitive replaces ad-hoc mpsc plumbing in the runtime so:
- progress events have monotonic ordering
- subscribers get watch-based backpressure
- close-as-cancel propagates through nested children
Pairs with #128 (in-transcript cards consume the mailbox stream).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
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