[codex] Add coverage checks

[66Ton99]

Summary

• Add PHP and JavaScript coverage scripts that generate Cobertura XML.
• Add a shared Cobertura coverage checker with line-coverage thresholds.
• Add a dedicated GitHub Actions Coverage job using PHP 8.3 with Xdebug and Node 22.
• Upload Cobertura reports to GitHub Code Quality with actions/upload-code-coverage@v1 when workflow permissions allow.
• Keep raw PHP/Jest coverage reports as workflow artifacts.
• Document the new local coverage commands in README.md and AGENTS.md.

Coverage thresholds

• PHP line coverage: minimum 50% (current local result: 51.36%).
• JavaScript line coverage: minimum 60% (current local result: 60.84%).

Validation

• php /tmp/jsfv-composer.phar validate --strict
• php /tmp/jsfv-composer.phar coverage
• npm run test:coverage
• php /tmp/jsfv-composer.phar test
• php /tmp/jsfv-composer.phar phpstan
• npm test
• git diff --check

Notes

• This follows GitHub Code Quality coverage docs: Cobertura XML, code-quality: write, PR head checkout, and actions/upload-code-coverage@v1.
• Coverage upload is skipped for fork pull requests by GitHub permission rules, but the Coverage check and artifacts still run on the PR. After merge/push inside the repository, GitHub can receive the uploaded reports.
• Existing npm dependency audit/deprecation notices remain, but tests and coverage checks pass.