add rule: never modify .gitignore

Author: s-celles

AGENTS.md

@@ -13,4 +13,5 @@
 - use authoritative sources (official documentation, standards bodies, .gov/.edu/.gouv.fr sites, peer-reviewed publications) and cite them with links
 - always verify that external links are currently valid and add "Vérifié le YYYY-MM-DD" (or "Verified on YYYY-MM-DD" for English content) after the link
 - never include {{< skill-buttons >}} in SKILL.md
-- update website lastUpdate date in hugo.toml (params.lastUpdate = "YYYY-MM-DD") when making changes
+- update website lastUpdate date in hugo.toml (params.lastUpdate = "YYYY-MM-DD") when making changes
+- never modify .gitignore