Add DDEV local development environment

[dd32]

Summary

• Replace wp-env approach with DDEV for local development, solving the bootstrap ordering issue where wp-env runs wp core install before custom constants are available
• DDEV gives full control over wp-config.php via disable_settings_management, so all constants are defined before WordPress bootstraps
• Use dynamic HTTP_HOST detection in sunrise files instead of static SITE_ID_CURRENT_SITE constants, making sunrise work in any environment
• Update CI to use DDEV for PHP tests across PHP 8.1, 8.4, and 8.5

Key changes

• .ddev/: DDEV project config (MariaDB 10.11, PHP 8.5, nginx-fpm), multisite nginx rewrites, wp-config template, idempotent setup script
• Sunrise files: Replace hardcoded SITE_ID_CURRENT_SITE/DOMAIN_CURRENT_SITE with get_domain_network_id using HTTP_HOST
• 0-aaa-constants.php: Fallback constants mu-plugin for environments where wp-config.php has not defined them yet
• CI: Replace MySQL service + setup-php with ddev/github-action-setup-ddev
• Bootstrap: Detect DDEV environment (/var/www/html) for correct paths

Test plan

• Verify CI passes on all PHP versions (8.1, 8.4, 8.5)
• ddev start successfully bootstraps WordPress with seed database
• Sites accessible: central.wordcamp.test, seattle.wordcamp.test/2023/, events.wordpress.test
• ddev exec phpunit runs tests
• Sunrise correctly routes requests for all three networks

Generated with Claude Code

[dd32]

This PR was supposed to go as dd32#3 as a comparison to dd32#2.

Both are an experimentation and not really ready for prime-time yet, I was looking for a more known route to ease LLM Agent testing.