Modernize project and migrate tests to sus

[ioquatix]

Summary

• modernize project metadata, workflows, and documentation layout
• migrate tests from RSpec to sus with covered integration
• restore the migration DSL to use using: with optional name: while forwarding remaining options
• add focused tests covering migration creation, execution, provider delegation, and name override

Testing

• bundle exec bake test
• COVERAGE=Summary bundle exec sus
• bundle exec rubocop