PR checklist enforcement

[softwareengineerprogrammer]

Description

Update PR checklist items and add config for required status check.

Testing & Verification

1. Unit tests added: N/A
2. Unit tests updated: N/A
3. Manual testing:
   1. Manually reviewed copy/markdown: https://github.com/softwareengineerprogrammer/GEOPHIRES/blob/pr-template_2026-05-30/.github/pull_request_template.md
   2. Enforce PR checklist verification test #161 -> https://github.com/softwareengineerprogrammer/GEOPHIRES/actions/runs/26691198840/job/78667780761?pr=161

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Change Checklist:

Observe the following checklist items if your change includes a new version (see versioning notes and version management docs).

Before merging:

1. I certify that I am a human and I have manually verified the correctness of the changes in this PR.
2. Add a CHANGELOG entry N/A

After merging:

1. Push tag(s) to GitHub: git push origin refs/tags/vM.m.P N/A
2. Post a change announcement to the GEOPHIRES Change Announcements Discussion N/A

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Self-reviewed in #160