ci: run WordPress Plugin Check on push to main only

[wzul]

What does this change?

Reduces GitHub Actions minutes by running the heavy WordPress Plugin Check only when changes land on main, not on every push/PR.

How

• plugin-check.yml (per-push/PR): keep the lightweight php-compatibility, phpunit, phpcs jobs; removed the plugin-check job and the build job that only fed it.
• plugin-check-main.yml (new): runs only wordpress/plugin-check-action on push: to main. Archives HEAD into ci-build/chip-for-gravity-forms, copies .wp-env.json, and runs plugin-check with build-dir/exclude-directories matching the previous usage.

Potential Risks & Review Items

• Plugin Check no longer gates PRs — issues will only surface after merge to main. Confirm that's acceptable for this plugin's release process.
• The build job removal assumes PHPUnit is pure PHP and never needs built JS assets (verified: phpunit stubs WP functions, phpcs/php-compat ignore assets/js/frontend).
• plugin-check-main.yml rebuilds via git archive + .wp-env.json; if built assets are ever required, they must be added to that workflow.