Initial conventions for AI agents and humans: AGENTS.md, SKILLS.md, instructions.md (#8478)

Not stable yet, but initial structure
https://agents.md
https://agentskills.io/specification
https://code.visualstudio.com/docs/copilot/customization/custom-instructions
https://code.visualstudio.com/docs/copilot/customization/agent-skills

.github/instructions/php.instructions.md

@@ -0,0 +1,37 @@
+---
+applyTo: "**/*.php"
+description: Editing PHP files
+---
+
+# PHP files
+
+* See minimum PHP version and available PHP extensions in [`composer.json`](../../composer.json)
+* Obey formatting rules defined in [`phpcs.xml`](../../phpcs.xml)
+* Automatic fixes can be done with:
+	```sh
+	composer run-script fix
+	# or (targeting more than just PHP)
+	make fix-all
+	```
+* Validation can be done with:
+	```sh
+	composer test
+	# or (targeting more than just PHP)
+	make test-all
+	```
+* Check [`composer.json`](../../composer.json) scripts for details about available individual commands.
+* For instance, running a single unit test can be done with:
+	```sh
+	composer run-script phpunit -- tests/app/Models/SearchTest.php
+	```
+
+## Autoloader
+
+* `spl_autoload_register` is defined in [`lib/lib_rss.php`](../../lib/lib_rss.php)
+
+Minimal example:
+
+```php
+require dirname(__DIR__) . '/constants.php';
+require LIB_PATH . '/lib_rss.php';	//Includes class autoloader
+```