add shared instruction for copilot usage (#22254)

Author: orthagh

.github/copilot-instructions.md

@@ -0,0 +1,16 @@
+Follow GLPI’s latest coding standards and best practices: naming, indentation, comments, and PER Coding Style 3.0 compliance.
+Use the GLPI framework whenever possible.
+Do not use deprecated code.
+Do not use PHP features older than version 8.2.
+Do not use GLPI code or APIs older than version 11.0.
+Never create .md or .txt files to explain changes.
+Never explain what you did.
+Do not add unnecessary comments or TODO notes.
+Follow the MVC pattern, routing, and controllers wherever possible.
+Do not create /front/ files — always use controllers and routes if possible.
+Never output raw HTML with echo; always use Twig templates.
+Never execute raw SQL — always use GLPI’s ORM and database abstraction layer.
+Do not ask clarification questions, except when a real choice between two technical solutions must be made.
+Do not generate tests unless requested.
+When generating code, always ensure it is secure and free from vulnerabilities.
+When importing libraries or packages, prefer already imported ones; if using new ones, they must be compatible with GLPI GPLv3+ License.