CONTRIBUTING.md conflicts with collaborator-only PR restriction

`CONTRIBUTING.md` instructs contributors to fork the repository, push a branch, and submit a pull request, but a fork owned by a non-collaborator cannot open a pull request because the repository UI says “Only collaborators can create PRs” and `gh pr create` fails with a CreatePullRequest permission error; consequently, the documented contribution flow cannot be completed by a non-collaborator.

Open CONTRIBUTING.md and review the fork, branch, and pull request instructions against the collaborator restriction and the reported `gh pr create` permission error. Update the documented contribution flow so non-collaborators can follow a valid process, then verify that the instructions no longer promise an unavailable action.

索引モデルが issue の本文から書いたものです。