[metrics] Add PhpMetrics command (#15)

[coisa]

Summary

• simplify the metrics command to delegate directly to PhpMetrics, remove the custom metrics loader/renderer layer, and execute PhpMetrics through the active PHP binary with deprecation noise suppressed
• align reports with optional metrics generation, keep preview-friendly output paths in the reports workflow, and include the metrics preview URL in the sticky PR comment
• refresh README, Sphinx docs, and the local .agents/skills/package-readme guidance to cover metrics badges, links, and the current command behavior
• update PHPUnit coverage for the new metrics and reports command flow

Validation

• ./vendor/bin/phpunit tests/Console/Command/MetricsCommandTest.php tests/Console/Command/ReportsCommandTest.php tests/Process/ProcessBuilderTest.php
• ./vendor/bin/phpunit tests
• composer dev-tools metrics -- --report-html=public/metrics
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/reports.yml"); YAML.load_file("resources/github-actions/reports.yml")'
• git diff --check

Notes

• the full PHPUnit run still reports an existing JoliNotif runner warning in this environment (signal 6 from the notification subscriber) after the suite completes successfully
• preview comments now include docs, coverage, and metrics links

Closes #15

[github-actions]

🚀 Preview is available for this pull request.

• Docs: https://php-fast-forward.github.io/dev-tools/previews/pr-98/
• Coverage: https://php-fast-forward.github.io/dev-tools/previews/pr-98/coverage/
• Metrics: https://php-fast-forward.github.io/dev-tools/previews/pr-98/metrics/