Infrastructure: pull request template (.github/pull_request_template.md)

[rosspeili]

Summary

Add a single, simple pull request template at .github/pull_request_template.md aligned with CONTRIBUTING.md and ARPA security practices.

Use default labels on PRs where relevant (documentation, enhancement, bug). Custom labels are out of scope until the issue-templates sub-issue follow-up.

Requirements

• Short, checklist-first (about one screen)
• No emojis
• Suggested sections:
  • Summary — what changed and why
  • Type of change — bug fix, docs, feature, refactor, security, chore
  • Related issues — Fixes # / Relates to #
  • Testing — import smoke, boot path, command spot-check, pytest if present
  • Documentation — README / docs/ if behavior or env vars changed
  • Security checklist — no secrets, no real kun.py or enrollment images; note Web3, biometric, or mail impact if applicable

Acceptance criteria

• .github/pull_request_template.md exists
• Wording matches the spirit of the current CONTRIBUTING.md checklist
• Links to CONTRIBUTING.md where helpful

Depends on

• Parent: Infrastructure: add .github directory