feat(generator): ship the docs-kit RuboCop cops from the gem (#22)

[mhenrixon]

Closes #22.

Problem

Both consumer sites carried a byte-identical hand-copied cop
(DocsKit/RenderComponentPreferred). Copy-paste cops drift and new sites never
got them. This upstreams the cops into the gem so a consuming site requires them
by gem path (two lines, scaffolded by the install generator) and deletes its
copies.

What ships

File	Purpose
lib/rubocop/cop/docs_kit/render_component_preferred.rb	The proven cop, upstreamed — prefers DocsUI::Code(...) over render DocsUI::Code.new(...)
lib/rubocop/cop/docs_kit/escaped_interpolation_in_heredoc.rb	New — steers \#{...} escapes in a double-quoted heredoc to a single-quoted delimiter (the recurring "escape tax")
lib/docs_kit/rubocop.rb	Require entry point — loads rubocop lazily (dev-time dep of the host, never a runtime dep of docs-kit)
config/rubocop/docs_kit.yml	Both cops enabled, scoped to app/views/docs/**/* by default

Wiring (automatic)

The install generator and docs-kit new inject two lines into the site's
.rubocop.yml, idempotently — created minimal when absent, merged (not
clobbered) into an existing rails new omakase config:

require:
  - docs_kit/rubocop
inherit_gem:
  docs-kit: config/rubocop/docs_kit.yml

RuboCop stays a development-time dependency of the host app. Every generated
site already has it via rubocop-rails-omakase.

Autocorrect-safety fixes (found by adversarial verification)

A multi-agent verification pass (probe against adversarial fixtures → independent
skeptical judges) surfaced three real defects, all now fixed and regression-tested:

• EscapedInterpolationInHeredoc — live sigil interpolation (HIGH). Ruby
  interpolates #@ivar and #$global in a double-quoted string too, not only
  #{...}. Live-detection was begin-child-only, so a heredoc whose live
  interpolation was #@name got its delimiter swapped and the interpolation
  silently frozen to literal text. Now any non-str dstr child counts as
  live. Verified on the real CLI: reports "fix by hand", leaves the file unchanged.
• EscapedInterpolationInHeredoc — inconsistent de-escaping (HIGH).
  Autocorrect stripped the backslash only from \#{ while switching to a
  single-quoted delimiter (which makes all backslashes literal), so a \#@name
  kept a backslash it used to consume. Now strips every escaped form
  (\#{, \#@, \#$).
• RenderComponentPreferred — overlapping corrections (MEDIUM). A kit render
  nested inside another kit render's block produced overlapping rewrite ranges
  (Parser::ClobberingError; the real CLI emitted a "this is a RuboCop bug"
  error). The correction now replaces only the render X.new(...) send, leaving
  the block in place so the inner render corrects independently.

Zeitwerk safety

lib/rubocop/** is outside the gem's loader dirs, and lib/docs_kit/rubocop.rb
(which defines RuboCop::Cop::DocsKit::*, not a DocsKit::Rubocop constant) is
loader.ignore-d. Specs assert the gem boots with no autoload registered for
DocsKit::Rubocop.

Test plan

• bundle exec rake — 489 examples, 0 failures, 96% coverage, RuboCop clean.
• 16 cop specs (expect_offense/expect_correction) including the sigil-interp
  live/de-escape cases and the nested-render clobber case.
• 6 generator specs: .rubocop.yml created when absent, merged into omakase,
  appends to an existing require list, idempotent on re-run.
• End-to-end against a fixture site + the dogfood docs/ app: cops fire,
  autocorrect, and respect the Include scope.

Note: the rubocop-rspec harness is scoped to the cop specs only. A global
require "rubocop/rspec/support" had shadowed the unrelated DocsKit::Registry
specs via CopHelper#registry under random ordering; fixed by including the
harness locally.

Out of scope

• Deleting the consumer-repo copies + switching them to the gem require (their
  repos, a follow-up).
• App-4's site-specific cops (they enforce a different UI kit).

https://claude.ai/code/session_01FPQb6z3YwcKRMbvoJhdxnX