Build/Test Tools: Smoke check installed sites with a shared composite action.

[adimoldovan]

Moves the upgrade smoke check into a composite action at .github/actions/smoke-check/, with the script in smoke-check.sh, and runs the same check after installation in install-testing.yml.

Trac ticket: https://core.trac.wordpress.org/ticket/66036

Follow-up to #13355, which added this check inline in reusable-upgrade-testing.yml.

The jobs check the action out into ci-tools with a sparse checkout, then call it by relative path, so the checks on this pull request run the script as it stands here.

The follow-up, #13389, changes the reference to WordPress/wordpress-develop/.github/actions/smoke-check@trunk and drops the checkout. Land it after this one. The release branches need that reference: 6.9, 7.0 and 7.1 call reusable-upgrade-testing.yml@trunk, so trunk's reusable workflow runs with github.ref set to the release branch, where .github/actions does not exist. Keeping the two steps apart also keeps the tag out of this pull request, where it would fail to resolve until the action is on trunk.

The workflow linter now covers the script too. actionlint shellchecks inline run: blocks but does not read composite actions, so the linter runs shellcheck over the scripts under .github/actions, and it triggers on changes to that directory.

Testing instructions

The Installation Tests and Upgrade Tests checks on this pull request run the step. Each single site row prints seven ok lines; multisite rows skip it.

To run the script by hand against a local site:

docker run -d --name wp-smoke-db -p 33061:3306 -e MYSQL_ROOT_PASSWORD=root -e MYSQL_DATABASE=test_db mysql:8.4
mkdir wp && cd wp
wp core download --version=nightly
wp config create --dbname=test_db --dbuser=root --dbpass=root --dbhost=127.0.0.1:33061
wp core install --url=http://localhost/ --title="Install Test" --admin_user=admin \
  --admin_password=password --admin_email=me@example.org --skip-email
RUNNER_TEMP=/tmp WP_ADMIN_USER=admin WP_ADMIN_PASSWORD=password \
  bash /path/to/wordpress-develop/.github/actions/smoke-check/smoke-check.sh

All seven checks print ok and the script exits 0. Note that it rewrites home and siteurl and edits wp-config.php, so the site is only good for the check afterwards.

To see it fail, run it again with a wrong WP_ADMIN_PASSWORD. The login check reports no authentication cookie, the script exits 1, and the server log goes to the output.

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Extracting the script into the composite action, the workflow changes, and iterative code review. I reviewed and tested the result.