Introduce an LLM policy for contributions

We need an LLM policy as we do get LLM based PRs from time to time from outside contributors - usually as a one-off contribution PR to the project. @Luap99 shared Podman's LLM policy here

https://github.com/podman-container-tools/community/blob/main/LLM_POLICY.md

We should adopt something like this and also create an AGENTS.md that explicitly mentions this LLM_POLICY file. As a majority of agentic tools look through the git repos for an AGENTS.md file this should at least let the vibecoder know what we don't want.