Document the one AI-assisted pull request limit

[acordivari]

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• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
• Have you explained what your changes do? Performance claims (e.g. "this is faster") must include brew benchmark results.
• Have you explained why you'd like these changes included, not just what they do?
• For bug fixes, have you given step-by-step brew commands to reproduce the bug?
• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

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AI-assisted contribution by Claude Code (Claude Opus 5, model claude-opus-5[1m]) of 100% of the work. brew style --changed, brew tests --changed, brew typecheck run by AI. Verified by the contributor: read the diff and confirmed the bullet matches CONTRIBUTING.md verbatim and sits in the same position in the list.

Built and tested locally on macOS 26.2 running arm64

Adds the concurrency limit that
CONTRIBUTING.md states but this page omits.

[MikeMcQuaid]

Passing on this. Documented elsewhere and enforced by GitHub.