refactor: modernise ruby_rbenv cookbook

[damacus]

Modernise the ruby_rbenv cookbook following the sous-chefs custom resource workflow.

Changes

• Add frozen_string_literal: true to all Ruby files
• Move test cookbook from test/fixtures/cookbooks/test to test/cookbooks/test
• Fix partial reference (_partial/git → _partial/_git) in system_install
• Remove EOL platforms; add current Ubuntu, Debian, AlmaLinux, Fedora platforms
• Add default kitchen suite exercising the primary system install workflow
• Align CI matrix with kitchen.yml suites and platforms
• Update Ruby versions in test recipes to 3.3.10, 3.4.9, 4.0.1
• Simplify package_deps.rb, removing EOL Debian version branches
• Add ChefSpec tests for all 8 resources and helper modules (63 examples, 0 failures)
• Add default InSpec suite with controls for the system install workflow
• Update Ruby versions in existing InSpec integration tests
• Move documentation to flat documentation/ structure
• Update README.md with current platform list, doc paths, and test cookbook path
• Add LIMITATIONS.md documenting install method, build deps, and platform/arch notes
• Remove stale test/unit specs

[github-actions]

Slowest examples

Top 10 slowest examples (1.07 seconds, 33.07% of total time)

Example	Description	Time in seconds
spec/unit/resources/user_install_spec.rb:38	rbenv_user_install with custom group is expected to create directory "/home/vagrant/.rbenv/plugins"	0.2418
spec/unit/resources/plugin_spec.rb:41	rbenv_plugin with user install is expected to sync git "Install ruby-build plugin"	0.16697
spec/unit/resources/gem_spec.rb:17	rbenv_gem action :install with default properties is expected to install gem_package "bundler"	0.0927
spec/unit/resources/system_install_spec.rb:14	rbenv_system_install with default properties (update_rbenv defaults to true) is expected to install package "["git", "grep"]"	0.08715
spec/unit/resources/user_install_spec.rb:16	rbenv_user_install with default properties is expected to install package "["git", "grep"]"	0.0868
spec/unit/resources/gem_spec.rb:53	rbenv_gem action :upgrade is expected to upgrade gem_package "bundler"	0.08575
spec/unit/resources/system_install_spec.rb:20	rbenv_system_install with default properties (update_rbenv defaults to true) is expected to nothing bash "Initialize system rbenv"	0.08511
spec/unit/resources/ruby_spec.rb:40	rbenv_ruby with user install is expected to run rbenv_script "rbenv install 3.4.9 (vagrant)"	0.07969
spec/unit/resources/user_install_spec.rb:20	rbenv_user_install with default properties is expected to create directory "/home/vagrant/.rbenv/versions"	0.07954
spec/unit/resources/user_install_spec.rb:50	rbenv_user_install with update_rbenv false is expected to checkout git "/home/vagrant/.rbenv"	0.07335