feat(rubocop): ship the docs-kit RuboCop cops from the gem

[mhenrixon]

Part of #8 (Phase 5).

Problem / Goal

Both consumer sites carry a byte-identical hand-copied cop — RuboCop::Cop::DocsKit::RenderComponentPreferred (enforces DocsUI::Code(...) kit-form over render DocsUI::Code.new(...)) at daisyui/docs/lib/rubocop/cop/docs_kit/... and phlex-reactive/docs/lib/rubocop/cop/docs_kit/.... It is already namespaced as if it were the gem's. Copy-paste cops drift and new sites don't get them.

Goal: the cops ship in the gem; a consuming site's .rubocop.yml gets two lines (scaffolded); the site copies get deleted.

Context (read these first)

• ~/Code/mhenrixon/daisyui/docs/lib/rubocop/cop/docs_kit/render_component_preferred.rb — the cop to upstream verbatim (it's proven; don't rewrite it).
• Both sites' .rubocop.yml require: lines — the wiring to replace.
• How gems ship cops: lib/rubocop/cop/docs_kit/ + a config/default.yml-style docs_kit.yml, loaded via require: [docs_kit/rubocop] + inherit_gem:.
• The heredoc-escape foot-gun from the audits (\#{...} scattered through docs pages) — the one NEW cop worth adding.

Decision

1. Move the cop into the gem at lib/rubocop/cop/docs_kit/render_component_preferred.rb, entry point lib/docs_kit/rubocop.rb (requires rubocop lazily — rubocop stays a development-time dependency of the HOST, not a runtime dep of docs-kit; document that sites need rubocop in their Gemfile, which every generated site has).
2. Ship config/rubocop/docs_kit.yml with both cops enabled and scoped to app/views/docs/**/* by default.
3. ONE new cop: DocsKit/EscapedInterpolationInHeredoc — flags \#{ inside a heredoc and suggests the single-quoted delimiter (<<~'RUBY') — kills the recurring escape tax found in every audited site. Autocorrect: unsafe (only when the heredoc contains no unescaped interpolation), otherwise suggestion-only.
4. Install generator + docs-kit new: append the require/inherit_gem lines to the site's .rubocop.yml (create a minimal one if absent), idempotently.
5. Consumer follow-up (their repos, not this one): delete the copies, switch to the gem require.

Alternatives rejected: a separate rubocop-docs_kit gem (release/version overhead for two cops; in-gem is the daisyui-gem-style precedent and the cop is already namespaced for it); porting the cop into a lint rake task (loses editor integration).

Implementation steps (TDD)

1. Cop specs first, spec/rubocop/cop/docs_kit/render_component_preferred_spec.rb (port the site's spec if one exists, else write: registers offense on render DocsUI::X.new(...) inside docs page paths, autocorrects to kit form, ignores non-DocsUI renders) and escaped_interpolation_in_heredoc_spec.rb (offense on \#{ in double-quoted heredoc; no offense in <<~'X'; safe-autocorrect case). Use rubocop-rspec's expect_offense harness; add rubocop to the gemspec development dependencies (it's already in the Gemfile for linting).
2. Move/implement the cops + lib/docs_kit/rubocop.rb + the shipped YAML.
3. Generator wiring (idempotent .rubocop.yml injection) + generator spec addition.
4. Run the new cops over the gem's own docs/ app; fix what they flag (dogfood).
5. README note + entries in the consumer re-sync guide (separate issue).

Verification gates

• bundle exec rspec spec/rubocop — green.
• bundle exec rubocop — no offenses (including the new cops against the dogfood app if wired there).
• In the dogfood app: .rubocop.yml requires the gem path and bundle exec rubocop runs the docs-kit cops.
• Zeitwerk safety: lib/rubocop/** is outside the gem's loader dirs — confirm loader.ignore isn't needed (it only pushes lib/docs_kit + app/components/docs_ui); a spec boots the gem and asserts no zeitwerk complaint.

Out of scope

• App-4's site-specific cops (raw_anchor/raw_button etc. — they enforce Zazu's own UI kit, not docs-kit's).
• More style cops (add only when a third real duplication appears).
• Editing the consumer repos.