feature: Add AGENTS.md and move skills

[FeodorFitsner]

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Describe the requested feature

We need to add basic instructions for AI agents.

Suggest a solution

Examples:
https://github.com/facebook/docusaurus/blob/main/AGENTS.md
https://github.com/openai/codex/blob/main/AGENTS.md

Other notes:

• project structure
• code style, commit comments (no AI mentioning)
• skills to .agents/skills?
• add instruction how to run unit tests
• use uv everywhere
• do we need CLAUDE.md -> AGENTS.md redirection?
• No double ticks in docstrings.

Screenshots

No response

Additional details

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