ci: add a vanilla v1.5 to v2 upgrade path workflow

[wychoong]

Adds a GitHub Actions workflow that runs the documented lunar:upgrade path against a throwaway Laravel 13 app: install lunarphp/core 1.5.0, migrate, switch to this checkout's core + upgrade packages, run php artisan lunar:upgrade, then php artisan migrate.

Matrix is PHP 8.5, Laravel 13, sqlite / mysql / postgres / mariadb. Core only — no admin panel or storefront. fail-fast is off so one driver does not stop the rest.

This is the missing coverage for the upgrade command itself. The existing upgrade Pest suite runs on SQLite :memory: against already-v2 fixtures; it does not install v1.5 and follow the published upgrade steps.

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[github-actions]

Thanks for the PR! I couldn't find a linked issue or Discussion in the description.

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