contributing: tighten AI usage policy (#18388)

contributing: tighten AI usage policy (#18388)

* contributing: tighten AI usage policy

* refactor AGENTS.md

* proofreading

* update contributing

* add claude.md

* add trailing newline

* add note about dishonest practices

* rm point about dishonest

* rm requirement watermarking

* add .gemini/settings.json

* allow initially AI-generated content

* revise

* Update CONTRIBUTING.md

Co-authored-by: Johannes Gäßler <redacted>
* improve

* trailing space

* Apply suggestions from code review

Co-authored-by: Johannes Gäßler <redacted>
* update

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Co-authored-by: Johannes Gäßler <redacted>