Add AI coding assistant policy to CONTRIBUTING.md, modelled on the
Linux kernel's coding-assistants.rst. Covers DCO/Signed-off-by
restrictions, licensing requirements, and the Assisted-by attribution
format.
Add AGENTS.md with codebase guidance for AI coding assistants.
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
