v0.19.0 — Plugin Dependencies in install-batch

Reports plugin dependencies after every install so the dashboard can auto-install missing wp.org dependencies. Closes the "Pro plugin requires free wp.org plugin" UX trap.

Added

• Installer::readPluginRequiresPlugins() — reads the Requires Plugins: header
• Installer::filterMissingDependencies() — narrows to slugs not installed/active locally
• install-batch response per-item gains:
  • requires_plugins — raw list from the header
  • missing_dependencies — slugs that need installation/activation

Wire contract

Purely additive. Old dashboards ignore the new keys; new dashboards (deckwp-app Plugin Dependencies sprint) auto-dispatch follow-up installs for the missing wp.org slugs.