Drop a message into another live agent's inbox.

Arguments:
- `to` (required) -- target label; non-existent errors.
- `content` (required) -- message text. Your label is attached.
- `delay` (optional) -- seconds to wait before delivery (tool returns immediately regardless). The delivered message automatically notes delay time.

Uses:
- Other agent coordination (they can send back to you too).
- Partial-result handoff.
- Delayed self-reminder (send to own label).
