Add repository-specific guidance for agents (AGENTS.md)

[qgallouedec]

What does this PR do?

Fixes # (issue)

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Note

Low Risk
Documentation-only change adding contribution/review guidance; no runtime or API behavior is affected.

Overview
Adds a new AGENTS.md guide describing repository conventions for contributors and reviewers, including expectations around main vs experimental code, how to handle paper-based implementations (update paper_index.md), and strict consistency requirements for intentionally duplicated trainer logic.

Also documents preferred simplicity principles and a required docstring format (including examples) plus a standard for linking to papers via huggingface.co/papers/....

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