Register on Packagist for Composer installation

[apermo]

Make the plugin installable via Composer (important for Bedrock, Sage, and modern WP setups).

• Update `composer.json` with correct package name (`apermo/surf-adminbar` or similar)
• Set `type` to `wordpress-plugin`
• Add `installer-name` for correct directory placement
• Register package on packagist.org
• Enable Packagist auto-update webhook (or use GitHub integration)
• Verify `composer require apermo/surf-adminbar` works

Packagist auto-updates from GitHub tags — the release workflow (#26) handles tagging.