[Feature]: Per-page noindex (robots meta) for unlinked or draft pages

Problem

There's no way to emit on an individual page. The is generated by the CMS, so a robots tag can't be added from the canvas, and there's no per-page toggle. I have two unlinked draft pages, kept in the CMS so they can go live later, that must not be indexed once the site is public. Unlinking keeps them out of navigation but not out of search once a crawler finds the URL.

Related bug report: (https://github.com/CoreBunch/Instatic/issues/521)

Proposed solution

Add a boolean field to the page panel, next to SEO title and SEO description: "Allow search engines to index this page," defaulting to on. When it's off, the publisher emits in the page head and excludes the page from any generated sitemap. Store it as a page cell (for example seoIndex) so it's also readable by plugins through api.cms.content and can be set in bulk.

Alternatives considered

A per-page "custom head" textarea that accepts raw tags would also solve this and covers other cases (canonical URLs, Open Graph overrides, verification tags). It's more flexible but easier to misuse; the toggle is the safer default and the two aren't mutually exclusive.

Area

Publishing