Extend browser artifact metadata persistence

[chubes4]

Summary

• Preserve optional caller-owned schema, kind, metadata, materialization, review hints, and apply-target metadata in canonical browser artifact bundles.
• Preserve browser session metadata/materialization under runtime metadata while keeping artifact content digest behavior file-content based.
• Expand smoke coverage for caller metadata, provenance passthrough, materialization/session metadata, text/binary files, and duplicate digests.

Tests

• 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

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Implementation and tests for the browser artifact persistence metadata extension; Chris remains responsible for review and merge.