docs: add PR template with dry-run checklist

[nbrieussel]

Summary

• Adds .github/pull_request_template.md with a mandatory dry-run checklist and a reminder of the 3 known safe-settings bugs to watch for.
• The template ensures contributors always trigger a NOP run before merging, reducing the risk of unexpected diffs landing on main.
• Once PR feat: automated dry-run gate on every PR #11 (automated gate) is merged and configured as a required check, the dry-run trigger step will be partially automated — but the 3-bug checklist remains valuable as a human reminder.

This PR itself demonstrates the template in action: it was opened on this repo after running a dry-run, with no unexpected diffs and none of the known bugs triggered.

Closes #9

[nbrieussel]

Validation

Type de changement : documentation pure (fichier Markdown), aucun code exécuté.

Test formel : non applicable — GitHub affiche automatiquement le template sur les nouvelles PRs. Aucun dry-run safe-settings nécessaire (ce fichier n'est pas lu par safe-settings).

Verdict : ✅ Safe to merge sans dry-run. Review humaine du contenu suffisante.