Add manual Prepublish integration tests

[MortenFriisSiteImprove]

Prepublish needs repeatable coverage for the draft-content and styling concerns raised in #64. This adds manual browser and WordPress integration tests, with readable Given/When/Then steps and failure screenshots.

• Verify never-published drafts and unpublished autosaves reach the plugin’s SDK handoff while anonymous visitors cannot retrieve the new content.
• Exercise same-origin and separate-origin Public URL settings through the toolbar and registered callback in Chromium and Firefox. Check preview parameters, PHP nonce validation, URL/token handoff, stylesheet references, inline styles, and cleanup.
• Add a disposable WordPress/PHP environment, pinned npm dependencies, scenario documentation, and downloadable HTML reports retained for 14 days.

Both new workflows use only workflow_dispatch. The optional plugin_ref input selects the plugin version independently of the test code; use refs/pull/64/head to test that fix. A blank input tests the workflow commit’s plugin, so the regression cases are expected to fail against current master until #64 is merged. The workflows record the resolved plugin commit and require no Siteimprove secrets.

Validation: 30 browser tests and 18 WordPress tests passed locally against #64 (a23af8519861f674d700cbe4fe183817f47458dc). The fresh-checkout run exposed a login-autofocus race; the tests now wait for WordPress’s initial focus before typing. Readiness and split-origin revision capture were revalidated in both browsers after that adjustment. Failure screenshot/report attachment was verified with temporary intentional failures in both browsers. The SDK is simulated; authenticated Siteimprove scans and report rerenders remain pending account access. Editor UI, additional WordPress/PHP versions, subdirectory installs and multisite are future coverage.

Production plugin files and existing release/deployment workflows are unchanged. The commit includes [skip ci] to avoid starting existing push/PR workflows during review. No GitHub workflow has been dispatched.