Add canonical browser artifact persistence

[chubes4]

Summary

• Adds wp-codebox/persist-browser-artifact as a REST-visible ability for parent sites to receive browser-produced files and persist a canonical WP Codebox artifact bundle.
• Stores mixed text/binary browser artifacts under files/browser/ with manifest entries, SHA-256 digests, changed-file metadata, review evidence, test-result placeholder, runtime metadata, and caller provenance.
• Reuses the existing artifact store/list/get bundle shape and validation rules, including safe relative paths, duplicate rejection, encoding validation, size limits, MIME type inference, and blocked server-executable extensions.

Verification

• php -l packages/wordpress-plugin/src/class-wp-codebox-artifacts.php
• php -l packages/wordpress-plugin/src/class-wp-codebox-abilities.php
• php -l tests/smoke-wordpress-plugin.php
• npm run wordpress-plugin-smoke
• npm run build

Issues

• Closes Add canonical browser artifact persistence from Playground back to parent #436
• Refs Capture browser artifact trees with binary file support #315
• Refs Expose reusable browser session recipe runner for product callers #437 / Add browser session recipe runner #446 for the separate browser recipe runner work

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the parent-side browser artifact persistence ability, canonical artifact bundle writer, and smoke coverage; Chris remains responsible for review and merge.