AI tool policy: add AGENTS.md

We have the AI policy in `contributing.rst` and the disclosure checkbox in the PR template, but no file that AI agents actually read.

Most coding agents pick up a root-level `AGENTS.md` automatically. So, the proposal is to add a minimal one that only points to existing docs (AI policy + disclosure, `CODING_STYLE.md`, `ReviewChecklist.md`, how to build and run the tests, Changelog expectations), plus a `CLAUDE.md` symlink. We would start with a minimal one, and we can improve it with coding style and stuff like that in future.

LLVM did the same recently:
- https://discourse.llvm.org/t/rfc-llvm-ai-tool-policy-agents-md/91712
- https://llvm.org/docs/AIToolPolicy.html (the "Handling violations" part could also go into contributing.rst)

Start by reading contributing.rst, the PR template, CODING_STYLE.md, ReviewChecklist.md, and the documented build, test, and Changelog guidance. Add a minimal root-level AGENTS.md that points to those existing resources, plus the proposed CLAUDE.md symlink; done means both files are present without duplicating policy content.

Written by the indexing model from the issue text.