This PR depends on [#31296](https://github.com/openai/codex/pull/31296)
for the canonical-to-legacy event mappings.
## Description
This PR makes the MultiAgentV2 spawn, message/follow-up, and interrupt
paths emit completed canonical `TurnItem::SubAgentActivity` items
instead of `SubAgentActivityEvent` directly.
App-server v2 now applies interrupted-agent thread-watch cleanup from
the canonical completed item and ignores the mapped legacy activity
event.
## Why
Sub-agent activity is separate from the v1 collab tool begin/end
lifecycle. Keeping it separate makes the v2 watcher side effect
reviewable without mixing in the larger collab tool-call migration.
## What changed
- Emit canonical sub-agent activity items from v2 spawn,
message/follow-up, and interrupt paths.
- Move missing-thread watcher cleanup onto canonical completed activity
items.
- Update focused app-server coverage to exercise canonical interrupted
activity.
Owen Lin
1bd9d841ca