chore: namespace v1 sub-agent tools

[jif-oai]

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.

[chatgpt-codex-connector]

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