chore: add a meaningful PR template

[ricogu]

Summary

Currently the repo has issue templates but no pull request template. Without one, PRs often lack the context reviewers need to understand what changed and why, slowing review and increasing the chance of regressions slipping through.

Proposed PR template

Create .github/PULL_REQUEST_TEMPLATE.md with:

• What changed — one-paragraph summary of the change
• Why / Motivation — link to issue or describe the problem being solved
• Type of change — checkbox list (bug fix / feature / refactor / chore / docs)
• Linked issue — Closes #<issue> so GitHub auto-closes on merge
• Testing — what was tested and how (unit, integration, manual E2E)
• Checklist — REUSE/license header on new files, docs updated if applicable, changelog or version notes if relevant

Acceptance criteria

• .github/PULL_REQUEST_TEMPLATE.md is created
• Template pre-fills the PR description on every new pull request
• Sections are concise — filling it in should take under two minutes
• Checklist items reflect this repo's conventions (REUSE, conventional-commit PR title, etc.)