## Why
The v1 sub-agent tools are a single tool family, but they were exposed
as separate flat function tools. This makes the model-visible surface
less clearly grouped and leaves the legacy names in the same flat
namespace as newer agent tooling.
## What
- Wraps the v1 `spawn_agent`, `send_input`, `resume_agent`,
`wait_agent`, and `close_agent` specs in the `multi_agent_v1` namespace.
- Registers the corresponding handlers with namespaced runtime tool
names.
- Updates tool-planning, deferred tool search, and sub-agent
notification tests to assert the namespace shape and child `spawn_agent`
lookup.
## Verification
- Updated `codex-core` coverage for the v1 multi-agent tool plan,
deferred tool search output, and sub-agent tool descriptions.
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