chore: remove SimpleCov code-coverage tooling

[tas50]

What

Removes the SimpleCov code-coverage machinery. It was opt-in (COVERAGE=1), never gated, never run in CI, and never consulted — so all it bought us was a dev dependency and an extra branch in the spec bootstrap.

Removed:

Location	What went
Gemfile	gem "simplecov", require: false from the :test group
spec/spec_helper.rb	the if ENV["COVERAGE"] block, require "simplecov", and SimpleCov.start
Rakefile	the desc "Run the unit tests with coverage reporting" / task :coverage task and its ENV["COVERAGE"] = "1"
.gitignore	the coverage artifact path
CONTRIBUTING.md	the rake coverage / COVERAGE=1 invocations and the "Coverage is reported but not gated" paragraph

There was no minimum_coverage / minimum_coverage_by_file enforcement, nothing under spec/support/ referenced SimpleCov, and no CI workflow uploaded or reported coverage — so nothing to remove there. The gemspec never carried the dependency either.

YARD documentation coverage is deliberately kept

The Rakefile also has desc "Report YARD documentation coverage" / task :doc_stats. That is documentation coverage, not code coverage, and it is untouched. Confirmed still present and working:

$ bundle exec rake -T | grep doc_stats
rake doc_stats              # Report YARD documentation coverage

$ bundle exec rake doc_stats
yard stats --list-undoc
Files:           5
Modules:         7 (    0 undocumented)
Classes:         8 (    0 undocumented)
Constants:       3 (    0 undocumented)
Attributes:      0 (    0 undocumented)
Methods:        51 (    0 undocumented)
 100.00% documented

Verification

Nothing left in the tree:

$ grep -rin 'simplecov' --exclude-dir=.git .
$ echo $?
1
$ grep -rn 'COVERAGE' --exclude-dir=.git .
$ echo $?
1

Test counts unchanged before and after the removal:

# before (origin/main)
595 runs, 1004 assertions, 0 failures, 0 errors, 0 skips

# after
$ bundle exec rake test
595 runs, 1004 assertions, 0 failures, 0 errors, 0 skips

Lint clean, on the same cookstyle line CI resolves (Gemfile.lock is gitignored, so CI takes the newest in ~> 9.0):

$ bundle exec cookstyle --version
Cookstyle 9.0.0
  * RuboCop 1.90.0

$ bundle exec cookstyle --chefstyle
Inspecting 21 files
.....................

21 files inspected, no offenses detected

CI on this PR has not finished at the time of writing.

Open PRs / merge order

No conflicts, and no merge order required. None of the open PRs touch Gemfile, Rakefile, or spec/spec_helper.rb:

• fix: build the rsync and credential-helper command lines as argv #422 (fix: shell argv) — lib/kitchen/driver/dokken.rb, lib/kitchen/transport/dokken.rb, spec/kitchen/driver/dokken_spec.rb, spec/kitchen/transport/dokken_spec.rb
• fix: honour the configured user_ns_mode #423 (fix: user_ns_mode) — README.md, lib/kitchen/driver/dokken.rb, spec/kitchen/driver/create_payload_spec.rb
• chore(main): release 2.25.0 #419 (release-please) — .release-please-manifest.json, CHANGELOG.md, lib/kitchen/driver/dokken_version.rb

This PR touches only Gemfile, Rakefile, spec/spec_helper.rb, .gitignore, and CONTRIBUTING.md, so it can merge in any order relative to the others.

(#424, the README docs PR, has already merged as 5a7474e.)