Closes#724
The previous policy pinned acceptance to a single past model version that
is no longer current SOTA, was not reliably detectable by reviewers, and
penalized contributors who honestly disclosed their tooling. Swap it for
a disclosure-only line — code is judged on merit, transparency is kept.
Add comprehensive contributor guidelines including code of conduct,
AI-generated code policy (GPT-5.2-codex only), PR best practices
(no mega PRs), bug report/feature request procedures, and getting
started instructions.
