V1 [INFRA] Pull request template

[austin-barrington]

Context

There is no .github/PULL_REQUEST_TEMPLATE.md. PR descriptions vary in quality. Reviewers must chase authors for basic information (test evidence, docs updates, changelog entry).

Deliverables

Create .github/PULL_REQUEST_TEMPLATE.md with sections:

• Description — what this PR does and why
• Checklist (checkbox list):
  • cargo fmt --all --check passes
  • cargo clippy --all-targets -- -D warnings passes
  • cargo test --lib passes
  • cargo test --test * passes
  • New features have documentation (user-guide or developer-guide)
  • New config keys have entries in docs/user-guide/configuration.md and config.toml.example
  • New metrics follow the naming convention (hyperbytedb_*)
  • Breaking changes are noted in the PR description
• Related issues — Closes #... or Refs #...
• Release notes — short summary for CHANGELOG

Priority

P1 — Developer workflow