Add pull request template with testing checklist

[smoparth]

Problem

No PR template exists. The repo has detailed contribution standards in CONTRIBUTING.md — Conventional Commits, type annotations, testing requirements, design patterns — but nothing reminds contributors at PR submission time. With 2000+ commits and active development, a lightweight checklist would reduce review friction.

What needs to happen

Add .github/PULL_REQUEST_TEMPLATE.md with a concise checklist that reflects the workflow in CONTRIBUTING.md.

Template should include

Sections:

• What — brief description of the change
• Why — link to issue or motivation

Testing checklist:

• hatch run test:test passes
• hatch run lint:check passes
• hatch run mypy:check passes
• New behavior has tests (unit and/or e2e as appropriate)

Standards checklist:

• Type annotations on all new/modified functions
• Docstrings on public APIs
• Conventional Commit format (feat:, fix:, refactor:, etc.)
• No trailing whitespace; files end with newline

Notes

• Keep it short. The full standards are in CONTRIBUTING.md — the template is a reminder, not a duplicate.
• The testing commands come directly from the Quick Reference table in CONTRIBUTING.md.
• e2e tests (hatch run e2e:run) are expensive and not always needed — don't make them a required checkbox, but mention them for relevant changes.

Definition of Done

• .github/PULL_REQUEST_TEMPLATE.md exists
• Includes What/Why sections
• Includes testing checklist with hatch commands
• Includes standards checklist (types, docstrings, commit format)
• Template renders correctly when opening a new PR on GitHub