[codex] Rename multi-agent v2 assign_task to followup_task

[jif-oai]

Summary

Renames the MultiAgentV2 turn-triggering tool from assign_task to followup_task so the exposed tool name better describes sending an additional task to an existing agent.

This updates the tool spec, handler/module names, registry wiring, default multi-agent v2 usage hints, and tests. Rollout trace classification keeps accepting legacy assign_task events so older traces still reduce correctly, while docs show the new tool name.

Test plan

• just test -p codex-core followup_task
• just test -p codex-core -E 'test(multi_agent_feature_selects_one_agent_tool_family) | test(multi_agent_v2_can_use_configured_tool_namespace) | test(code_mode_only_can_expose_namespaced_multi_agent_v2_as_normal_tools)'
• just test -p codex-rollout-trace
• just fix -p codex-core
• just fix -p codex-rollout-trace

Notes: just fmt ran cargo fmt but failed in the Python ruff phase because the local environment could not resolve hatchling>=1.27.0 from the configured internal registry. A full just test -p codex-core also hit unrelated environment-sensitive integration failures involving missing spawned test binaries/sandbox behavior; the changed multi-agent spec/handler tests passed in the filtered runs above.

[jif-oai]

@codex review

[chatgpt-codex-connector]

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[jif-oai]

For the record, we don't care about legacy