Problem: No guidance for AI coding agents working in the Vim
repository.
Solution: Add AGENTS.md to the repository documenting build
and test commands, repository layout, commit format,
C and Vim9-script conventions, test conventions, help
file style, and release policy, so that AI agents can
produce patches that match project expectations.
closes: #20039
Signed-off-by: Christian Brabandt <cb@256bit.org>