docs request: surface agents.max_threads in official config reference

Summary

Please consider documenting agents.max_threads (default/min/value semantics) in official configuration docs.

Why

This is a practical knob for multi-agent concurrency tuning, but it is not obvious to users unless they inspect source.

Prior PR context

I attempted an in-repo docs update in PR #12534, and understand docs now live in the official docs source and contribution policy is invitation-only.