Add PixelPerfect quality standard and GitHub CI

[pixelhed]

Summary

• Adds `pixelperfectat/coding-standard ^0.1.0` to `require-dev` (public Packagist — no extra `repositories` entry needed)
• Adds `phpunit/phpunit ^9.6` to `require-dev`; removes `rector/rector` from require-dev (conflicts with `magento/magento-coding-standard ^33` which pins `rector ^0.17`)
• Scaffolds `phpcs.xml` (`<rule ref="PixelPerfect"/>`, `ignore_warnings_on_exit`), `phpstan.neon.dist`, `phpunit.xml.dist`, `.github/workflows/ci.yml`
• CI is inline (mirrors ci-templates@0.1.3 steps) — cross-repo reusable workflow calls are blocked on GitHub Free plan
• Runs `phpcbf` on existing code (5 auto-fixes in 3 files); 29 residual warnings suppressed via `ignore_warnings_on_exit`
• Generates phpstan baseline (22 pre-existing errors: League\Csv v8 API against v9, missing return types, wrong property type in CmsHelper)
• Adds `Test/Unit/.gitkeep` so phpunit finds the directory and exits 0 on empty suite (PHPUnit 9.x exits 0 when no tests run)
• Excludes `.sync-conflict-` files from phpcs, phpstan, and git tracking

Adoption findings

Item	Finding
PHP constraint	Added `^8.3` (was: none)
phpcbf auto-fixes	5 fixes in 3 files
Residual phpcs warnings	29 (all suppressed — `ignore_warnings_on_exit`)
phpstan baseline	22 errors (League\Csv v8→v9 API mismatch, missing types)
Tests present	No — empty suite exits 0 on PHPUnit 9.x
rector conflict	rector/rector ^2.2 in require-dev conflicts with magento-coding-standard ^33 (pins rector ^0.17) — removed from require-dev
Reusable workflow	❌ Cross-repo `uses:` blocked on GitHub Free plan — inlined steps instead

Action needed to make CI green

The org secrets `MAGENTO_PUBLIC_KEY` / `MAGENTO_PRIVATE_KEY` are set but returning HTTP 401 from `repo.magento.com`. The module requires 63 packages from `repo.magento.com` (via `magento/module-cms: "*"` transitive deps). Update the secrets with valid Magento Marketplace credentials:

```bash
gh secret set MAGENTO_PUBLIC_KEY --org pixelperfectat
gh secret set MAGENTO_PRIVATE_KEY --org pixelperfectat
```

Once the secrets are valid, re-run the CI. Commitlint is already green.

CI status

• commitlint — PASSING
• quality/validate — blocked (auth)
• quality/lint — blocked (auth)
• quality/cs — blocked (auth)
• quality/stan — blocked (auth)
• quality/test — blocked (auth)