docs: add architecture context for contributors and agents

[mickaelbegon]

Closes #1098.

Summary

• add concise repository instructions in AGENTS.md;
• add a CLAUDE.md discovery entry point requested in the issue;
• document the observed OCP architecture and scientific invariants;
• provide a task-to-source/test context map for contributors and coding agents;
• reference the material from the contribution guide and PR checklist.

Scope

Documentation only: no runtime dependency, API, numerical behavior, CI rule, or architectural refactor is introduced.

Validation

• git diff --check
• referenced local files and Markdown links checked

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This change is