## Summary
- align the default multi-agent v2 root and subagent hints with the
evaluated prompt guidance for direct collaboration-tool calls, parallel
delegation, and shared workspaces
- keep the current `interrupt_agent` tool name and existing
concurrency-hint placement, with the explicit no-spawn instruction last
- document the context tradeoff between `fork_turns="none"` and
`fork_turns="all"` in the v2 `spawn_agent` description
- extend the focused prompt and tool-surface tests
## Why
The evaluated multi-agent prompt includes operational guidance that is
missing from the current Codex defaults. This applies that guidance to
the current tool surface without restoring stale `close_agent` or
duplicated concurrency wording.
## User impact
Multi-agent v2 receives clearer instructions about when and how to
parallelize work, how agent workspaces interact, and how `fork_turns`
affects subagent context. The existing default opt-out behavior remains
in place.
## Testing
- `just fmt`
- `just test -p codex-core
multi_agent_v2_default_usage_hints_use_configured_thread_cap`
- `just test -p codex-core
multi_agent_feature_selects_one_agent_tool_family`
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