feat: add CHANGELOG.md entry checkbox to PR template

[tschm]

Background

The rhiza PR template (pull_request_template.md) does not include a reminder to add a CHANGELOG.md entry. basanos added this checklist item and it has been a useful nudge to keep the changelog current.

Current State

## Checklist

- [ ] Commit messages follow the Conventional Commits format
- [ ] Documentation updated if behaviour changed
- [ ] `make deptry` passes (no unused or missing dependencies)

Proposed Change

Add a CHANGELOG.md line to the checklist:

## Checklist

- [ ] Commit messages follow the Conventional Commits format
- [ ] `CHANGELOG.md` entry added
- [ ] Documentation updated if behaviour changed
- [ ] `make deptry` passes (no unused or missing dependencies)

Motivation

The changelog is the primary artifact consumers read when upgrading. A PR checklist item is a low-friction reminder that costs nothing when skipped on trivial changes, but prevents the common case where the changelog simply gets forgotten.

Reference

Observed in jebel-quant/basanos .github/PULL_REQUEST_TEMPLATE.md.