Suggestion: add a pull request template

### Summary

The delta repository does not have a pull request template, which means contributors submit PRs with inconsistent formatting, missing checklists, and vague descriptions. Adding a PR template would improve review quality and reduce maintainer overhead.

### Steps to reproduce

1. Go to the repository: https://github.com/dandavison/delta
2. Look in `.github/` — there is no `pull_request_template.md` and no `PULL_REQUEST_TEMPLATE/` directory
3. Check the repo root and `docs/` — also no template
4. Check if `dandavison/.github` exists as an org-level fallback — it doesn't

### Expected behavior

The repository should provide a PR template so contributors know what information to include (e.g. issue linkage, testing steps, change summary).

### Actual behavior

No PR template exists anywhere in the repository or at the org level.

### Suggested approach

Create `.github/pull_request_template.md` with at minimum:
- Issue linkage (`Fixes #`)
- Type of change (bug fix / feature / refactor / docs)
- Brief summary of changes and motivation
- Testing approach and results
- Contributor checklist (self-tested, no unrelated changes, etc.)

Reference: [delta has issue templates in `.github/ISSUE_TEMPLATE/`](https://github.com/dandavison/delta/tree/main/.github/ISSUE_TEMPLATE) so the precedent for contributor structure already exists.

### Environment

Repository: `dandavison/delta`
Branch: `main`

Start by reviewing the existing files in .github/ISSUE_TEMPLATE/ to match the repository’s contributor-facing structure. Create .github/pull_request_template.md with issue linkage, change type, summary and motivation, testing details, and the listed contributor checklist. Done means the template exists at that path and covers each requested section.

Written by the indexing model from the issue text.