fix: pollora:status reports the slug the attribute registers

[ogorzalka]

Closes #298.

Problem

SystemInfoCollector (behind pollora:status and the dashboard) derived each post type and taxonomy slug from the class name (kebab(class_basename)), ignoring #[PostType('…')] / #[Taxonomy('…')]. It also skipped the 20-character cut discovery applies to a derived slug, and ignored the attribute's plural label.

Change

• The slug rule lives on the attributes: PostType::resolveSlug($className) and Taxonomy::resolveSlug($className) — explicit slug, else kebab-case of the class name cut to 20 characters.
• PostTypeDiscovery / TaxonomyDiscovery register with it (their private generateSlug() copies are gone).
• The collector reads the attribute by reflection and uses the same method; label falls back to the attribute's plural before the class name.

Tests

• resolveSlug() for both attributes (explicit, derived, 20-char cut).
• Collector test with the exact class from the issue (#[PostType('synthese-presse', …)] class SyntheseDePresse) — fails on the old code (synthese-de-presse).
• On a real site (pollora-test): pollora:status lists test_case for #[PostType('test_case')] class TestCase — matching wp post-type list — where the old code showed test-case.
• Full suite 1372 green, PHPStan 0, Rector clean.