Fix GitHub Actions failures: code quality, tests, and linting

- Fix shellcheck warnings in bin/install-wp-tests.sh (quote variables, fix command -v usage)
- Remove trailing spaces in .github/workflows/phpunit.yml
- Add phpmd.xml to exclude camelCase checks for WordPress naming conventions
- Update composer.json to use phpmd.xml configuration
- Remove trailing commas in .eslintrc.js for Codacy compliance
- Add .markdownlint.json to configure markdown linting rules
- Improve Cypress test reliability with increased timeouts
- Update loginAsAdmin command with better error handling
- Make plugin activation checks more robust in Cypress tests

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composer.json

@@ -53,7 +53,7 @@
         "phpcbf": "vendor/bin/phpcbf --standard=phpcs.xml",
         "phpcbf:simple": "vendor/bin/phpcbf --standard=phpcs-simple.xml",
         "phpstan": "vendor/bin/phpstan analyse --level=5 .",
-        "phpmd": "vendor/bin/phpmd . text cleancode,codesize,controversial,design,naming,unusedcode --exclude vendor,node_modules,tests,bin,build,dist",
+        "phpmd": "vendor/bin/phpmd . text phpmd.xml --exclude vendor,node_modules,tests,bin,build,dist",
         "test": "vendor/bin/phpunit",
         "lint": ["@phpcs", "@phpstan", "@phpmd"],
         "fix": ["@phpcbf"]