Consolidate multi-agent v2 communication sends

[bolinfest]

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.

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)

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• Log multi-agent communication lifecycle #30872
• -> Consolidate multi-agent v2 communication sends #30867