Prior to this commit, CONTRIBUTING.md delegated build instructions
and code style to wiki pages, which are planned for removal.
This commit inlines the build, code style and testing guidelines
into CONTRIBUTING.md, aligns them with the current build and
Checkstyle rules, and documents the commit message conventions, the
security policy and the policy on AI-assisted contributions. It also
adds a concise AGENTS.md extract for coding agents, and updates the
README to link to the Build from Source section.
Closes gh-37372
Signed-off-by: Sébastien Deleuze <sdeleuze@users.noreply.github.com>
