CI: build, test and lint on every push

[LukasNiessen]

Standard Ruby CI. Run the test suite, the linter, and the dogfooding architecture tests.

[TristanKruse]

Baseline CI landed on main in commit 4a8940c. It runs the full RSpec/RuboCop gate and gem build on Ruby 3.3, 3.4, and 4.0 on Linux plus Ruby 4.0 on Windows. Keeping this issue open because the requested dogfooding architecture tests can only be added after the files fluent API exists.