core: preserve thread activity through completion

[winston-openai]

Summary

• hold the exact thread-activity reservation from accepted turn dispatch through terminal delivery
• preserve the reservation across replacement and interrupt-to-restart transitions
• keep V1 completion watchers bound to the exact child session and hold activity through V1/V2 parent notification
• enumerate descendants from the stable ThreadId ancestry so unloading an intermediate agent cannot hide a live grandchild

This is the final core layer of the thread-bound idle shutdown stack and depends on #31349.

Test plan

• CARGO_INCREMENTAL=0 just test -p codex-core 'accepted_submission_can_start_before_queued_shutdown' 'activity_reservations_are_atomic_and_survive_session_teardown' 'try_start_turn_if_idle_rejects_active_turn_without_injecting' 'interrupt_waits_for_idle_turn_start_to_install_task' 'replacement_holds_thread_activity_through_startup' 'task_finish_retries_trigger_mail_that_arrives_before_active_turn_clears' 'task_finish_emits_thread_idle_lifecycle_after_active_turn_clears' 'idle_shutdown_claim_waits_for_submissions_and_blocks_automatic_turns' 'spawn_child_completion_notifies_parent_history' 'multi_agent_v2_completion_queues_message_for_direct_parent' 'completion_watcher_notifies_parent_when_child_is_missing' 'idle_parent_cannot_shutdown_while_unpublished_child_waits_for_approval'
• CARGO_INCREMENTAL=0 just fix -p codex-core
• just fmt
• independent principal review: 10 focused checks passed

Stack

• core: make idle thread shutdown atomic #31304
• core: track thread publication lifecycle #31333
• core: make thread activity lifecycle atomic #31338
• core: serialize thread turn transitions #31349
• this PR

[winston-openai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: c9d6f4f444

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