feat(onboarding): add public completion gate

[guisaliba]

Closes #349

Summary

• expose the tenancy-agnostic OnboardingCompletionGate contract with isCompleted and completedAt
• keep Eloquent and onboarding models behind an internal adapter resolved through the service container
• require an exact user, onboarding type, and completed status before opening the gate
• return the persisted completion transition timestamp without exposing internal models

Tests

• completed, in-progress, paused, rejected, missing, and mismatched-type gate behavior
• persisted completion timestamp plus null behavior for incomplete and missing onboardings
• contract resolution through the Laravel container

Verification

• vendor/bin/pest --compact app-modules/onboarding/tests/Feature/OnboardingCompletionGateTest.php
• make test (942 tests, 3,001 assertions)
• npm run build
• make test-rector
• make test-pint
• make phpstan

[BrunaDomingues]

LGTM!

[arthurbazzz]

LGTM