Add baseline GitHub Actions checks

[jdevalk]

Summary

• add WordPress coding standards checks
• add PHP linting across supported PHP versions
• add Composer diff and security audit workflows
• add a Playground PR preview workflow

Why

This repo had no GitHub Actions coverage yet, so this adds a practical baseline without overengineering.

[ilicfilip]

@jdevalk , we usually add Playgroud Preview button in a separate comment (example), so should we do it here as well for consitency?

It would also resolve the thing Claude complains about (I have not verified it):

1. playground-preview.yml triggers on edited — this can infinite-loop / spam.
The workflow uses mode: append-to-description, which edits the PR description. The trigger includes types: [..., edited]. A pull_request: edited event fires when the description changes — so the workflow's own description edit can re-trigger the workflow. The WordPress/action-wp-playground-pr-preview action uses <!-- wp-playground-preview:start/end --> markers (visible in this PR's body) to update-in-place rather than append repeatedly, which usually breaks the loop — but edited also fires on title edits and any manual body edit, causing needless re-runs. Recommend dropping edited from the trigger list unless there's a specific reason for it. At minimum, confirm the action is idempotent on its own edits (it appears to be, given the markers).