doc: state that external PRs are closed automatically This PR updates `CONTRIBUTING.md` and the PR template to say that the repository only accepts PRs from the Lean FRO team and a small set of regular contributors, and to point everyone else to the issue tracker. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This is the repository for Lean 4.
See Install Lean.
Please read our Contribution Guidelines first.
See Building Lean.