Integrate fizzy-saas as vendored gem at saas/

- Update Gemfile.saas to use path: "saas" instead of GitHub source
- Update config/database.yml to reference local saas/ directory
- Update bin/setup to source saas/bin/setup directly
- Remove redundant dotfiles (ruby-version, gitignore, rubocop, github workflows)
- Add saas/db exclusions to root rubocop config

config/database.yml

@@ -1,6 +1,6 @@
 <%
   config_path = if Fizzy.saas?
-    gem_path = Gem::Specification.find_by_name("fizzy-saas").gem_dir
+    gem_path = Rails.root.join("saas").to_s
     File.join(gem_path, "config", "database.yml")
   else
     File.join("config", "database.#{Fizzy.db_adapter}.yml")