Gemfile.saas evals Gemfile, so shared gems should have identical versions
in both lockfiles. This adds bin/bundle-drift to detect and fix drift:
* `bin/bundle-drift check` compares shared gem versions
* `bin/bundle-drift correct` seeds Gemfile.lock from Gemfile.saas.lock
and re-locks, letting Bundler prune SaaS-only gems while preserving
shared versions
Adds drift check to bin/ci and GitHub CI. Corrects existing drift.