feat: surface multi-agent thread limit in spawn description

[jif-oai]

Summary

• Thread agent_max_threads into ToolsConfig and SpawnAgentToolOptions.
• Render the configured max_concurrent_threads_per_session value in the MultiAgentV2 spawn_agent description.
• Cover the description text in codex-tools unit tests and codex-core tool spec tests.

Validation

• just fmt
• cargo test -p codex-tools
• cargo test -p codex-core spawn_agent_description
• git diff --check

Notes

• cargo test -p codex-core was also attempted, but unrelated environment-sensitive tests failed with the active local environment. Examples: approvals reviewer defaults observed AutoReview instead of User, request-permissions event tests did not emit events, and proxy-env tests saw http://127.0.0.1:50604 from the active proxy environment.

[jif-oai]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

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