Gate multi-agent v2 tools independently of collab

[jif-oai]

Why

multi_agents_v2 is meant to be independently gated from the older collab feature. The tool registry still treated the collaboration-style agent tools as collab-only, so enabling multi_agents_v2 without collab omitted the v2 agent tools. Review and guardian sub-sessions also need to keep agent spawning disabled even when the outer session has multi_agents_v2 enabled.

What changed

• Include the collab-backed agent tools when either multi_agents_v2 or collab is enabled.
• Explicitly disable multi_agents_v2 for review and guardian review sub-sessions, matching the existing spawn_csv and collab restrictions.
• Add a registry test that enables multi_agents_v2, disables collab, and verifies the v2 agent tools are present while legacy send_input and resume_agent remain hidden.

Testing

• Added test_build_specs_multi_agent_v2_does_not_require_collab_feature.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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