Add a pull-request template that reminds contributors to sign off (DCO)

[Maxerns]

Why this is a good first issue

CONTRIBUTING.md requires a Developer Certificate of Origin (`git commit -s`) and says unsigned commits cannot be merged. First-time PRs on #136 and #137 both missed it, because nothing in the GitHub PR form mentions it. A template is one markdown file.

What to do

Add .github/pull_request_template.md with, in this order:

1. A one-line summary of the change.
2. `Closes #` (issue number).
3. A short test plan (what you ran, or "docs only").
4. A checkbox: "I signed off every commit (`git commit -s` / Developer Certificate of Origin)."

Keep it short. British English, no em dashes. No Copilot/CLA theatre, no extra checklists.

Out of scope

Do not add a DCO GitHub App, a CI check, or a CODEOWNERS file. Do not rewrite CONTRIBUTING.md beyond a single sentence pointing at the template if you want.