Add a pull request template under .github/

[hanneshapke]

The repo has a bug-report issue template (.github/ISSUE_TEMPLATE/10_bug_report.yml) and a semantic commit guide, but no pull request template. Contributors have to guess what reviewers want to see.

What to add

Create .github/PULL_REQUEST_TEMPLATE.md (or .github/pull_request_template.md) with sections:

• Summary — what changed and why
• Test plan — checklist of how to verify
• Changeset — reminder to run npm run changeset (link to docs/04-release-management.md)
• Related issues — "Closes #..."

Hints

• Keep it lightweight — too long a template discourages PRs.
• Mirror the style of the existing bug report template.

Difficulty: good first issue, ~30 min.