ci: migrate yum tests to Policyfile

[damacus]

Summary

• replace Berkshelf resolution with Policyfile
• declare local and external cookbook sources explicitly
• update ChefSpec and Copilot setup for Policyfile

Verification

• chef install Policyfile.rb
• cookstyle
• embedded Chef rspec (29 examples, 0 failures)
• markdownlint-cli2, actionlint, yamllint
• Kitchen not run: local Docker socket unavailable

[github-actions]

Slowest examples

Top 10 slowest examples (0.94 seconds, 45.69% of total time)

Example	Description	Time in seconds
spec/unit/resources/globalconfig_spec.rb:48	yum_globalconfig on Amazon Linux is expected to render file "/tmp/yum.conf"	0.15192
spec/unit/resources/dnf_module_spec.rb:111	dnf_module enable when module is already enabled is expected to enable dnf_module "test:1.0"	0.1094
spec/unit/resources/dnf_module_spec.rb:45	dnf_module install when module is not installed is expected to install dnf_module "test:1.0"	0.09032
spec/unit/resources/globalconfig_spec.rb:148	yum_globalconfig with custom properties is expected to render file "/tmp/yum-full.conf"	0.08868
spec/unit/resources/globalconfig_spec.rb:149	yum_globalconfig with custom properties is expected to render file "/tmp/yum-full.conf"	0.08686
spec/unit/resources/globalconfig_spec.rb:150	yum_globalconfig with custom properties is expected to render file "/tmp/yum-full.conf"	0.08616
spec/unit/resources/globalconfig_spec.rb:18	yum_globalconfig with default properties is expected to render file "/tmp/yum.conf"	0.0849
spec/unit/resources/globalconfig_spec.rb:146	yum_globalconfig with custom properties is expected to render file "/tmp/yum-full.conf"	0.08478
spec/unit/resources/globalconfig_spec.rb:147	yum_globalconfig with custom properties is expected to render file "/tmp/yum-full.conf"	0.08144
spec/unit/resources/globalconfig_spec.rb:49	yum_globalconfig on Amazon Linux is expected to render file "/tmp/yum.conf"	0.08115