docs: add link to AGENTS.md in CONTRIBUTING.md (#5121)

* add link to AGENTS.md in CONTRIBUTING.md

Assisted-by: Claude Sonnet 4.6

* Update CONTRIBUTING.md: fix broken link

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Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>