Set up PHPStan static analysis (Apermo setup)

[apermo]

Add PHPStan using apermo/phpstan-wordpress-rules.

Once PR #28 is merged (v0.2.0), this package bundles everything:

• phpstan/phpstan-strict-rules
• szepeviktor/phpstan-wordpress (WP stubs)
• phpstan/phpstan-deprecation-rules
• 13 custom WordPress rules (security, i18n, WP API misuse, code quality)

Single dependency

{
    "require-dev": {
        "apermo/phpstan-wordpress-rules": "^0.2"
    }
}

All extensions auto-register via phpstan/extension-installer.

Configuration (phpstan.neon)

parameters:
    level: 6
    paths:
        - apermo-adminbar.php
        - classes

That's it — the Apermo package handles the rest (strict rules config, WP stubs, custom rules).

Tasks

• Wait for apermo/phpstan-wordpress-rules v0.2.0
• Add apermo/phpstan-wordpress-rules ^0.2 as dev dependency
• Add phpstan/extension-installer to allowed Composer plugins
• Create minimal phpstan.neon (level 6, paths only)
• Add "analyse": "phpstan analyse" to Composer scripts
• Add to CI workflow (Set up CI workflow #25)
• Fix all reported issues before merge