fix: migrate to Policyfile

[damacus]

Summary

• add Policyfile.rb and remove Berksfile
• switch ChefSpec to chefspec/policyfile
• update Copilot dependency install to chef install Policyfile.rb
• update Copilot instructions to reference Policyfile dependency setup

Verification

• chef install Policyfile.rb
• cookstyle --no-server --cache-root /private/tmp/rubocop_cache
• /opt/homebrew/bin/markdownlint-cli2 "**/*.md"
• env GEM_HOME=/private/tmp/chef-gem-home GEM_PATH=/opt/chef-workstation/embedded/lib/ruby/gems/3.1.0:/private/tmp/chef-gem-home /opt/chef-workstation/embedded/bin/rspec
• git diff --check
• kitchen list
• KITCHEN_LOCAL_YAML=kitchen.dokken.yml kitchen list

[github-actions]

Slowest examples

Top 10 slowest examples (0.53 seconds, 74.7% of total time)

Example	Description	Time in seconds
spec/unit/resources/vagrant_gem_spec.rb:14	vagrant_gem with action remove is expected to remove gem_package "vagrant"	0.10759
spec/unit/resources/vagrant_spec.rb:54	vagrant with appimage is expected to install package "openssh-client"	0.09756
spec/unit/resources/vagrant_plugin_spec.rb:44	vagrant_plugin with action remove uninstalls the plugin	0.04569
spec/unit/resources/vagrant_spec.rb:56	vagrant with appimage is expected to extract archive_file "/tmp/d20260702-2395-ilepat/vagrant.zip"	0.04283
spec/unit/resources/vagrant_spec.rb:41	vagrant with plugins is expected to install vagrant_plugin "vagrant-ohai"	0.04276
spec/unit/resources/vagrant_spec.rb:24	vagrant with default properties is expected to install dpkg_package "vagrant"	0.04243
spec/unit/resources/vagrant_spec.rb:42	vagrant with plugins is expected to install vagrant_plugin "vagrant-berkshelf"	0.04186
spec/unit/resources/vagrant_gem_spec.rb:15	vagrant_gem with action remove is expected to remove chef_gem "vagrant"	0.03968
spec/unit/resources/vagrant_spec.rb:23	vagrant with default properties is expected to create remote_file "/tmp/d20260702-2395-cj6a67/vagrant.deb"	0.03942
spec/unit/resources/vagrant_spec.rb:55	vagrant with appimage is expected to create remote_file "/tmp/d20260702-2395-c0et5o/vagrant.zip"	0.03875