Add instructions (not only) for Claude AI agent

[adamkudrna]

Completely auto-generated, no manual edits.

[bedrich-schindler]

I just quickly looked at it and did not see anything that might be wrong. As we expect this to be changed a bit when devcontainers are introduced, I would not spent much time on it. It seems quite compact so it should not polute project. I am just thinking (generally, not specifically for this project) whether we want to have Claude-specific things because we might use not just Claude. I am thinking about maintaining just generic AGENTS.md and symlinking CLAUDE.md to it. But Copilot for example can read CLAUDE.md as well https://docs.github.com/en/enterprise-cloud@latest/copilot/tutorials/coding-agent/get-the-best-results#adding-custom-instructions-to-your-repository. Aside from symlinking, there is native file include using @ anthropics/claude-code#6235 (comment).

[adamkudrna]

@bedrich-schindler As agreed in person, we will stick with CLAUDE.md for now as the only common config understood by all major AI agents. We may switch to AGENTS.md in the future when anthropics/claude-code#6235 is resolved. Potential structuring into individual "configs" (Git workflow, coding style, testing, …) is also possible when we see fit.