prepare-wordpress: PHP test scaffolding, phpcs.xml, ESLint; plugin 1.1.0

[soderlind]

• PHP test scaffolding: phpunit.xml.dist, tests/bootstrap.php, tests/TestCase.php (Brain Monkey), created in the composer phase.
• composer.json now written as a file (no composer init with user strings) — fixes SKILL/planner inconsistency + hardening.
• New eslint phase: @wordpress/eslint-plugin, .eslintrc.json, lint:js.
• Committed phpcs.xml ruleset; composer lint script -> phpcs. New references/linting-setup.md.
• Plugin bumped to 1.1.0 with CHANGELOG.

Rebuilt plugin + discovery index; detect/plan scripts updated and smoke-tested.