docs(config): clarify default agent thread limit

[middleDuckAi]

What?

• clarify the agents.max_threads default in the config source-of-truth comment
• regenerate codex-rs/core/config.schema.json

Why?

• current runtime config resolution defaults agents.max_threads to Some(6)
• the generated schema still said omitting the field enforced no limit
• the duplicate thread for #8702 points at docs-site content, but this repo-owned schema mismatch still exists on current main

How?

• update the AgentsToml::max_threads doc comment in codex-rs/core/src/config/mod.rs
• regenerate the checked-in schema fixture

Testing

• cargo run -p codex-core --bin codex-write-config-schema
• cargo test -p codex-core config_schema_matches_fixture -- --nocapture

Refs openai#10517.