ci: add GitHub PR template — standard checklist for every pull request

[BigBen-7]

Description

PRs are opened without a consistent checklist, leading to missed steps (no tests, no CHANGELOG update, clippy warnings).

What to do

Create .github/PULL_REQUEST_TEMPLATE.md:

## What does this PR do?
<!-- One paragraph summary -->

## Closes
Closes #

## Checklist
- [ ] Tests added or updated
- [ ] `cargo test` passes locally
- [ ] `cargo fmt --all --check` passes
- [ ] `cargo clippy -- -D warnings` passes
- [ ] `CHANGELOG.md` updated under [Unreleased]
- [ ] `docs/error-codes.md` updated if new panic strings added
- [ ] `docs/events-reference.md` updated if new events emitted

PR requirement

• Closes #[issue_id] — Timeframe: 24 hours