Three personalized-fork PRs (#155, #162, #165) were filed against
upstream by accident in one week - GitHub points new fork PRs at the
upstream repo by default, and nothing warned about it at the moment of
filing. Adds a PR template with the heads-up in the compose box (plus
the review norms the process asks for anyway) and one sentence in
CONTRIBUTING's fork section naming the mechanism.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
