chore: add AGENTS.md to guide AI-assisted contributions (#5089)

* add AGENTS.md to guide AI-assisted contributions

Assisted-by: Claude Sonnet 4.6

* Apply suggestions from code review

Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>

* fix AGENTS.md lint command description

Assisted-by: Claude Sonnet 4.6

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