pr-build-zip: optional 'Try this PR in a live test site' link (central enrolment)

[lihsaa591]

Summary

The PR build comment can now include a link that opens a fresh WordPress (Playground, in the browser) with that PR's build installed and activated, logged in as admin. Aimed at QA verification and at support reproducing issues.

• No change in the ~90 caller repos. Repos are enrolled centrally in config/preview-link-repos.json. A repo not listed gets no link and no behaviour change. Enrolled today: only themegrill/user-registration.
• Why not on by default for everyone: about 47 of 88 repos are add-ons (33, need their base plugin) or *-pro (14, open unlicensed), and themes need type: theme. A link that looks right but fails is worse than none. preview-link: true|false and preview-* inputs let a caller override.
• Cannot break the build comment: the new steps are continue-on-error; the script validates every value (the link is posted to a PR) and exits 0 with no output for non-enrolled repos.
• Only the primary ZIP is previewed. Link and zip expire together (~30 days).

One-time infra change (already done)

themegrill-pr-artifacts CORS now allows GET, HEAD from https://playground.wordpress.net. Verified: that origin is allowed, another origin is rejected.

Testing

• npm test: 68/68 (new: link builder + validation, enrolment resolution, running the script the way the workflow does).
• Real run: throwaway branch in themegrill/user-registration (caller changed only to @feat/pr-preview-link + preview-tools-ref), manual dispatch -> success; link generated from that run's zip (HEAD 200, correct CORS); the real "Compose comment body" step rendered locally shows the link in the comment. Branch deleted.
• A person opened a generated link: plugin installs and activates.

Not verified

• A live build of a NON-enrolled repo (covered by unit tests only).
• requires (base-plugin install) and type: theme in a browser.
• Because this edits a workflow shared by ~90 repos, a mistake would break every build. That is why the real run above was done before this PR; the sparse tooling checkout adds a few seconds to every build.

Docs

CHANGELOG entry; onboard-pr-build-zip-repo skill has an enrolment/testing section.