Expose artifact apply preflight contract

[chubes4]

Summary

• Adds wp-codebox/apply-artifact-preflight and wp codebox artifacts preflight-apply so parent control planes can load, verify, approve-check, and receive the apply adapter payload without parent-side git/PR/deploy mutation.
• Reuses the new preflight helper inside apply_approved() so validation, patch filtering, digest metadata, and payload construction stay in one WP Codebox-owned path.
• Improves requested corrupt-bundle resolution so digest/missing-patch failures surface as validation errors instead of misleading not-found responses.

Testing

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

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafting the upstream preflight contract, targeted smoke coverage, and verification commands; Chris remains responsible for review and merge.