Update PR guidance to link and auto-close resolved issues

[danielfromearth]

The PR guidance is a bit vague on how to link issues to a PR (as discussed during Collaboration Cafe, 14 April 2026).

Let's be more explicit and link to the list of GitHub keywords for auto-closing issues, in:

• PR template
• Contributor PR Guide

[danielfromearth]

For the PR template:

Existing language:

• PR body contains links to related and resolved issues (e.g. closes #1)

Proposed language:

• Use GitHub keywords (e.g., Closes #1) in the PR body to auto-close resolved issues, and standard references (e.g., Relates to #2, Addresses #3) for ongoing work.