Playground PR review support for two-factor

[masteradhoc]

What?

Add a WordPress Playground preview button to pull requests.

Fixes #774

Why?

Makes it easier to test plugin changes without a local environment — reviewers can click a button directly from the PR to try the changes in a live Playground instance.

How?

Two new workflows:

• build-plugin-zip.yml — builds the plugin and uploads it as an artifact on every PR
• pr-playground-preview.yml — triggers after the build, exposes the artifact publicly, and appends a Playground preview button to the PR description

Testing Instructions

1. Open a pull request
2. Wait for both Build Plugin Zip workflow to complete
3. download the zip created over the workflow over Actions tab in github repo
4. extract it and install the "two-factor" zip inside
5. test the changes made in this PR

Whats not testable yet: PR Playground Preview.
The reason is a GitHub limitation with workflow_run — workflows using that trigger must exist on the default branch to work. Since pr-playground-preview.yml only exists on this feature branch (not yet merged to master), GitHub ignores it.

Changelog Entry

Added - Playground preview button on pull requests for easier testing.

[github-actions]

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Co-authored-by: masteradhoc <masteradhoc@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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