chore: exclude dev files from dist archives; ensure license metadata

Packaging hygiene (phpnomad-wide initiative): tests, CI config, and tool
configs are export-ignored so dist installs stop shipping fixtures and
stubs that can redeclare framework interfaces in consumers' analysis
scope. LICENSE file ensured.

Author: alexstandiford

.gitattributes

@@ -0,0 +1,6 @@
+/tests export-ignore
+/.github export-ignore
+/phpunit.xml export-ignore
+/.php-cs-fixer.dist.php export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore