Add GitHub Actions CI (Travis config is dead, phpunit/cs-fixer configs unused)

[onetechgroupll]

From the SDK review 2026-09-15.

Evidence: the only CI config is .travis.yml, which effectively no longer builds public repos. Meanwhile phpunit.xml.dist and .php-cs-fixer.dist.php exist but run nowhere, and test/ holds 95 generator stubs.

Fix: minimal GitHub Actions workflow — composer install, phpunit, php-cs-fixer dry-run. Second step: replace/extend the stubs with the round-trip smoke tests from clients/_smoke/ in the platform repo.