docs: recommend Conventional Commits for commit messages in branches #58

CONTRIBUTING.md requires Conventional Commits only for the PR title (it becomes the squash commit in main); commit messages inside a branch are free-form.

Recommend (not enforce) the same format for commits in branches, so the history of a PR reads the same way as main. CI keeps checking only the PR title: with squash and merge the branch commits do not get into main.

Note: the repository settings currently allow only Rebase and merge, while CONTRIBUTING.md describes Squash and merge. The settings need to be switched to squash (a maintainer does it in Settings → General → Pull Requests).