chore: add a pull request template

[peb-peb]

Summary

The repo has no pull request template. Add one so contributors describe their changes consistently and reviewers get the context they need.

Tasks

• Add .github/PULL_REQUEST_TEMPLATE.md.
• Include sections for: Description, Related issue (Closes #), Type of change, How was this tested?, and a Checklist.
• Checklist items: ran pnpm format / pnpm format:check, pnpm build passes, self-review done, docs updated if needed.

Suggested skeleton

## Description
<!-- What does this PR do and why? -->

## Related issue
Closes #

## Type of change
- [ ] Bug fix
- [ ] New feature
- [ ] Docs
- [ ] Refactor / chore

## How was this tested?

## Checklist
- [ ] `pnpm build` passes
- [ ] Ran `pnpm format:check`
- [ ] Self-reviewed the diff
- [ ] Updated docs where relevant

Acceptance criteria

• Opening a PR pre-fills the description with this template.