Why

MultiAgentV2 follow-up messages are delivered to agents as assistant-authored InterAgentCommunication envelopes. When followup_task used interrupt: true, the interrupted-turn guidance was still persisted as a contextual user message, so model-visible history made a system-generated interruption boundary look user-authored.

This keeps interruption guidance consistent with the rest of the v2 inter-agent message stream while preserving the legacy marker shape for non-v2 sessions.

What changed

• Make interrupted_turn_history_marker feature-aware.
• Record the interrupted-turn marker as an assistant OutputText message when Feature::MultiAgentV2 is enabled.
• Keep the existing user contextual fragment for non-v2 sessions.
• Apply the same feature-aware marker to interrupted fork snapshots.
• Add coverage for the live followup_task interrupt path and the helper-level v2 marker shape.

Testing

• cargo test -p codex-core multi_agent_v2_followup_task_interrupts_busy_child_without_losing_message -- --nocapture
• cargo test -p codex-core multi_agent_v2_interrupted_marker_uses_assistant_output_message -- --nocapture
• cargo test -p codex-core interrupted_fork_snapshot -- --nocapture