## Why
Multi-agent v2 communications currently use separate outbound paths:
direct messages, follow-up tasks, and completion results go through
`send_inter_agent_communication`, while a spawn's initial message goes
through the generic input submission path. That split makes it difficult
to add complete communication lifecycle logging in one place.
This refactor makes `submit_inter_agent_communication` the common sink
for those paths, preparing the follow-up observability work discussed in
[#30516](https://github.com/openai/codex/pull/30516).
## What changed
- Routed all current outbound `InterAgentCommunication` paths in
`AgentControl`—direct messages, follow-up tasks, completion results, and
multi-agent v2 spawn initial messages—through
`submit_inter_agent_communication`.
- Centralized the actual submission and last-task-message bookkeeping
there, providing one place for the follow-up PR to instrument
communication creation and successful enqueue.
- Left non-communication input handling and the multi-agent v1 spawn
flow unchanged.
## Testing
- `just test -p codex-core 'agent::control::tests::'` (51 passed)
- `just test -p codex-core
'suite::subagent_notifications::encrypted_multi_agent_v2_spawn_sends_agent_message_to_child'`
(passed)
---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/30867).
* #30872
* __->__ #30867
Michael Bolin
a98a21798c