Add the AGENTS.md details that cost time to rediscover

[tas50]

AGENTS.md deliberately points at DEVELOPER_GUIDE.md and WRITING_RULES.md instead of duplicating them. This adds only the things that are in none of the three and that you find by hitting them.

What was added

A cop's name comes from its module nesting, not its file path. rake validate_config walks the constants under RuboCop::Cop::Chef, so the directory is convention. The guides write the path as <department>, which holds for six of the eight directories and not for these:

Directory	Department
lib/rubocop/cop/chef/deprecation/	Chef/Deprecations
lib/rubocop/cop/chef/redundant/	Chef/RedundantCode
lib/rubocop/cop/inspec/deprecation/	InSpec/Deprecations
lib/rubocop/cop/chefstyle/ruby/	Chef/Ruby, validated against config/chefstyle.yml

rake generate_cops_yml_documentation is not idempotent on current Ruby. On a clean checkout it rewrites 12 files with whitespace-only changes: Psych 5.3 stopped emitting a trailing space after an empty examples: or version_added: key, and those files were generated before that. An agent following the documented "run this after adding a cop" step gets 12 files of churn mixed into their diff. Documented with a way out (git checkout -- docs-chef-io/ after staging your own file) rather than fixed, since regenerating is a whitespace-only commit across the generated tree — happy to send that separately if wanted.

CI runs linelint over every file, rake style only over Ruby. Layout/TrailingEmptyLines covers .rb, but a config/cookstyle.yml entry or a Markdown file missing its trailing newline passes rake and fails on the PR.

Smaller ones: the negative-case rule is enforced by spec/negative_case_coverage_spec.rb, so it fails as a repo-wide list of paths rather than in your own spec; Chef/Effortless is slated for removal so new cops shouldn't go there; and new cop files need no require, since lib/cookstyle.rb globs the tree.

Labels can't be applied from a fork. gh pr create --label opens the PR and then fails on AddLabelsToLabelable, which reads like the whole command failed and invites a retry that would open a second PR.

Verification

• bundle exec rake exits 0 — 1239 examples, 0 failures; validate_config clean. The 11 refactor-severity offenses rake style reports are pre-existing on main (identical with this change stashed) and don't affect the exit code.
• Every claim checked against the tree: the department mapping from module nesting, the generator drift by running it and reverting, the linelint gap by confirming cookstyle --list-target-files returns 0 Markdown/YAML files out of 567.
• AGENTS.md ends in exactly one newline.

Labels

I don't have write access, so I couldn't apply these: Aspect: Documentation, Expeditor: Skip All.

Unrelated things noticed, not touched

Per the Scope section of AGENTS.md, flagging rather than fixing:

1. tasks/prof.rake:14 — Rake::Task[:run] is a global lookup, not namespaced, so rake prof:slow_cops and rake prof:walk abort with Don't know how to build task 'run'. Should be Rake::Task['prof:run']. DEVELOPER_GUIDE.md documents both as working.
2. Rakefile — the RSpec task sets spec.pattern = FileList['spec/cop/**/*.rb'], but specs live in spec/rubocop/cop/. It is inert today: the empty FileList makes RSpec::Core::RakeTask omit --pattern, so rspec falls back to spec/ and runs everything. Worth knowing that a naive fix to spec/rubocop/cop/**/*_spec.rb would silently drop spec/negative_case_coverage_spec.rb.
3. cookstyle.gemspec — required_ruby_version = '>= 2.7' while CI tests 3.1, 3.4, and 4.0.