Docs: clarify related issues in PR template

[SilverSupplier]

Summary

• Rename the PR template issue-link block to Related Issues.
• Move related issue fields directly below Summary so reviewers can check issue context before validation details.
• Split issue references into Closes, Related, and Follow-up fields so PR authors can separate resolved, contextual, and future work issues.

Related Issues

• Closes:
• Related:
• Follow-up:

Verification

• node scripts/validate-doc-state.mjs
• GitHub checks observed: validate-docs, test, build, CodeQL passed before the section-order update; re-running after latest push.
• npm test; Not run locally, template-only change.
• npm run build; Not run locally, template-only change.
• Unity EditMode tests: Not run, template-only change.
• Unity PlayMode tests: Not run, template-only change.

Notes

• Scope: PR template issue-link wording and placement only.
• Out of scope / follow-up: changing existing PR bodies.
• Risk notes: low; this only changes default PR body text.

Review Checklist

• This PR changes user-facing UI or behavior.
• This PR changes recognizer behavior.
• This PR changes docs, task state, or project process.
• This PR changes ML artifacts, datasets, generated outputs, or capture/export behavior.
• This PR changes logging, survey fields, privacy, or research data handling.
• This PR needs screenshots or a short playtest note.

Screenshots / Playtest Notes

Not applicable; no UI/gameplay change.