Add PHP 8.2 strict types, PHPCS, and PHPStan linting

[PatelUtkarsh]

Summary

• Bumps minimum PHP to >=8.2 and adds declare(strict_types=1) to all PHP files
• Adds PHPCS (WordPress Coding Standards 3.0) and PHPStan (level 6 with WordPress extensions) as dev dependencies
• Adds GitHub Actions CI workflow that runs both linters on push to master and on PRs

Changes

Composer

• Min PHP >=8.2, type changed from package to library
• Dev deps: wpcs ^3.0, phpcompatibility-wp ^2.1, phpstan ^2.0, szepeviktor/phpstan-wordpress ^2.0, dealerdirect/phpcodesniffer-composer-installer ^1.0
• Scripts: phpcs, phpcbf, phpstan, lint

Code Quality Fixes

• All esc_html__() calls now include the shadow-taxonomy text domain
• File and class docblocks added to all files
• @param tags added for WP-CLI command $args/$assoc_args parameters
• Return type fixes in get_associated_post() and get_related_post_by_slug() (now properly returns WP_Post|false)
• Simplified post_type_already_in_sync() — removed always-true isset checks and unreachable code
• Intentional phpcs:ignore comments for slow DB queries and unused public API parameters

New Files

• phpcs.xml.dist — WordPress coding standards config
• phpstan.neon.dist — PHPStan level 6 config (CLI file excluded from analysis due to missing WP-CLI stubs)
• .github/workflows/lint.yml — CI pipeline

Breaking Changes

Adding declare(strict_types=1) means PHP will enforce strict type checking for function arguments in these files. Callers passing incorrect types (e.g. string where int is expected) will now get TypeError instead of silent coercion.