Replace deployment workflows with CI checks

[pparage]

What changed

• remove the validation and production deployment workflows
• add CI for PHP tests, Pint, Composer validation/audit, npm audit, and the Vite production build
• add pull-request dependency review with least-privilege permissions and pinned action commits
• add weekly Dependabot updates for GitHub Actions, Composer, and npm
• remove the obsolete test that inspected deleted deployment workflow files

Why

Deployments are performed manually, so GitHub Actions should validate changes without holding deployment credentials or modifying servers.

Validation

• 221 PHPUnit tests passed (481 assertions; 4 skipped, 1 incomplete)
• Laravel Pint passed
• Composer and npm audits found no vulnerabilities
• Vite production build passed
• actionlint passed
• YAML parsing and diff checks passed