Curate the Cortext block picker

[priethor]

What

Part of #85.

Cortext now keeps the block picker to the set of blocks we want in documents. Cortext blocks get their own category, locked header blocks still work behind the scenes, and blocks added by other plugins do not show up unless we add them to the list.

Why

Gutenberg's default picker is too broad for this editor. It includes site blocks, feeds, Query Loop pieces, and post-context blocks that either do not belong in Cortext or would render badly here. Plugin-added blocks can add more clutter, so this gives us a picker we control.

How

• Passing the same allowed-block settings into Canvas and row editors.
• Registering the Cortext category before Cortext blocks load.
• Keeping locked header blocks available to Cortext while hiding them from the picker.

Allowed blocks:

• Cortext: cortext/data-view, cortext/document-icon, cortext/document-cover, cortext/document-properties.
• Text and structure: core/paragraph, core/heading, core/list, core/list-item, core/quote, core/pullquote, core/verse, core/code, core/preformatted, core/html, core/math, core/footnotes, core/details, core/post-title.
• Media: core/image, core/gallery, core/video, core/audio, core/file, core/cover, core/media-text.
• Layout: core/columns, core/column, core/group, core/separator, core/spacer.
• Interactive: core/button, core/buttons, core/accordion, core/accordion-item, core/accordion-heading, core/accordion-panel, core/social-link, core/social-links.
• Utility: core/table, core/embed, core/table-of-contents, core/icon.
• Document metadata: core/post-date, core/post-time-to-read.
• Reusable: core/pattern, core/block.

Testing Instructions

1. Open a Cortext page or row editor.
2. Open the block inserter.
3. Confirm document blocks and Cortext blocks are available.
4. Search for blocks like Latest Posts, RSS, Navigation, and Query Loop, and confirm they do not appear.
5. Confirm an existing document still shows its title/header normally.